Hu Chunming / FFNvDecoder

  @chapter Syntax
  @c man begin SYNTAX
  
  This section documents the syntax and formats employed by the FFmpeg
  libraries and tools.
  
  @anchor{quoting_and_escaping}
  @section Quoting and escaping
  
  FFmpeg adopts the following quoting and escaping mechanism, unless
  explicitly specified. The following rules are applied:
  
  @itemize
  @item
  @samp{'} and @samp{\} are special characters (respectively used for
  quoting and escaping). In addition to them, there might be other
  special characters depending on the specific syntax where the escaping
  and quoting are employed.
  
  @item
  A special character is escaped by prefixing it with a @samp{\}.
  
  @item
  All characters enclosed between @samp{''} are included literally in the
  parsed string. The quote character @samp{'} itself cannot be quoted,
  so you may need to close the quote and escape it.
  
  @item
  Leading and trailing whitespaces, unless escaped or quoted, are
  removed from the parsed string.
  @end itemize
  
  Note that you may need to add a second level of escaping when using
  the command line or a script, which depends on the syntax of the
  adopted shell language.
  
  The function @code{av_get_token} defined in
  @file{libavutil/avstring.h} can be used to parse a token quoted or
  escaped according to the rules defined above.
  
  The tool @file{tools/ffescape} in the FFmpeg source tree can be used
  to automatically quote or escape a string in a script.
  
  @subsection Examples
  
  @itemize
  @item
  Escape the string @code{Crime d'Amour} containing the @code{'} special
  character:
  @example
  Crime d\'Amour
  @end example
  
  @item
  The string above contains a quote, so the @code{'} needs to be escaped
  when quoting it:
  @example
  'Crime d'\''Amour'
  @end example
  
  @item
  Include leading or trailing whitespaces using quoting:
  @example
  '  this string starts and ends with whitespaces  '
  @end example
  
  @item
  Escaping and quoting can be mixed together:
  @example
  ' The string '\'string\'' is a string '
  @end example
  
  @item
  To include a literal @samp{\} you can use either escaping or quoting:
  @example
  'c:\foo' can be written as c:\\foo
  @end example
  @end itemize
  
  @anchor{date syntax}
  @section Date
  
  The accepted syntax is:
  @example
  [(YYYY-MM-DD|YYYYMMDD)[T|t| ]]((HH:MM:SS[.m...]]])|(HHMMSS[.m...]]]))[Z]
  now
  @end example
  
  If the value is "now" it takes the current time.
  
  Time is local time unless Z is appended, in which case it is
  interpreted as UTC.
  If the year-month-day part is not specified it takes the current
  year-month-day.
  
  @anchor{time duration syntax}
  @section Time duration
  
  There are two accepted syntaxes for expressing time duration.
  
  @example
  [-][@var{HH}:]@var{MM}:@var{SS}[.@var{m}...]
  @end example
  
  @var{HH} expresses the number of hours, @var{MM} the number of minutes
  for a maximum of 2 digits, and @var{SS} the number of seconds for a
  maximum of 2 digits. The @var{m} at the end expresses decimal value for
  @var{SS}.
  
  @emph{or}
  
  @example
  [-]@var{S}+[.@var{m}...]
  @end example
  
  @var{S} expresses the number of seconds, with the optional decimal part
  @var{m}.
  
  In both expressions, the optional @samp{-} indicates negative duration.
  
  @subsection Examples
  
  The following examples are all valid time duration:
  
  @table @samp
  @item 55
  55 seconds
  
  @item 12:03:45
  12 hours, 03 minutes and 45 seconds
  
  @item 23.189
  23.189 seconds
  @end table
  
  @anchor{video size syntax}
  @section Video size
  Specify the size of the sourced video, it may be a string of the form
  @var{width}x@var{height}, or the name of a size abbreviation.
  
  The following abbreviations are recognized:
  @table @samp
  @item ntsc
  720x480
  @item pal
  720x576
  @item qntsc
  352x240
  @item qpal
  352x288
  @item sntsc
  640x480
  @item spal
  768x576
  @item film
  352x240
  @item ntsc-film
  352x240
  @item sqcif
  128x96
  @item qcif
  176x144
  @item cif
  352x288
  @item 4cif
  704x576
  @item 16cif
  1408x1152
  @item qqvga
  160x120
  @item qvga
  320x240
  @item vga
  640x480
  @item svga
  800x600
  @item xga
  1024x768
  @item uxga
  1600x1200
  @item qxga
  2048x1536
  @item sxga
  1280x1024
  @item qsxga
  2560x2048
  @item hsxga
  5120x4096
  @item wvga
  852x480
  @item wxga
  1366x768
  @item wsxga
  1600x1024
  @item wuxga
  1920x1200
  @item woxga
  2560x1600
  @item wqsxga
  3200x2048
  @item wquxga
  3840x2400
  @item whsxga
  6400x4096
  @item whuxga
  7680x4800
  @item cga
  320x200
  @item ega
  640x350
  @item hd480
  852x480
  @item hd720
  1280x720
  @item hd1080
  1920x1080
  @item 2k
  2048x1080
  @item 2kflat
  1998x1080
  @item 2kscope
  2048x858
  @item 4k
  4096x2160
  @item 4kflat
  3996x2160
  @item 4kscope
  4096x1716
  @item nhd
  640x360
  @item hqvga
  240x160
  @item wqvga
  400x240
  @item fwqvga
  432x240
  @item hvga
  480x320
  @item qhd
  960x540
  @item 2kdci
  2048x1080
  @item 4kdci
  4096x2160
  @item uhd2160
  3840x2160
  @item uhd4320
  7680x4320
  @end table
  
  @anchor{video rate syntax}
  @section Video rate
  
  Specify the frame rate of a video, expressed as the number of frames
  generated per second. It has to be a string in the format
  @var{frame_rate_num}/@var{frame_rate_den}, an integer number, a float
  number or a valid video frame rate abbreviation.
  
  The following abbreviations are recognized:
  @table @samp
  @item ntsc
  30000/1001
  @item pal
  25/1
  @item qntsc
  30000/1001
  @item qpal
  25/1
  @item sntsc
  30000/1001
  @item spal
  25/1
  @item film
  24/1
  @item ntsc-film
  24000/1001
  @end table
  
  @anchor{ratio syntax}
  @section Ratio
  
  A ratio can be expressed as an expression, or in the form
  @var{numerator}:@var{denominator}.
  
  Note that a ratio with infinite (1/0) or negative value is
  considered valid, so you should check on the returned value if you
  want to exclude those values.
  
  The undefined value can be expressed using the "0:0" string.
  
  @anchor{color syntax}
  @section Color
  
  It can be the name of a color as defined below (case insensitive match) or a
  @code{[0x|#]RRGGBB[AA]} sequence, possibly followed by @@ and a string
  representing the alpha component.
  
  The alpha component may be a string composed by "0x" followed by an
  hexadecimal number or a decimal number between 0.0 and 1.0, which
  represents the opacity value (@samp{0x00} or @samp{0.0} means completely
  transparent, @samp{0xff} or @samp{1.0} completely opaque). If the alpha
  component is not specified then @samp{0xff} is assumed.
  
  The string @samp{random} will result in a random color.
  
  The following names of colors are recognized:
  @table @samp
  @item AliceBlue
  0xF0F8FF
  @item AntiqueWhite
  0xFAEBD7
  @item Aqua
  0x00FFFF
  @item Aquamarine
  0x7FFFD4
  @item Azure
  0xF0FFFF
  @item Beige
  0xF5F5DC
  @item Bisque
  0xFFE4C4
  @item Black
  0x000000
  @item BlanchedAlmond
  0xFFEBCD
  @item Blue
  0x0000FF
  @item BlueViolet
  0x8A2BE2
  @item Brown
  0xA52A2A
  @item BurlyWood
  0xDEB887
  @item CadetBlue
  0x5F9EA0
  @item Chartreuse
  0x7FFF00
  @item Chocolate
  0xD2691E
  @item Coral
  0xFF7F50
  @item CornflowerBlue
  0x6495ED
  @item Cornsilk
  0xFFF8DC
  @item Crimson
  0xDC143C
  @item Cyan
  0x00FFFF
  @item DarkBlue
  0x00008B
  @item DarkCyan
  0x008B8B
  @item DarkGoldenRod
  0xB8860B
  @item DarkGray
  0xA9A9A9
  @item DarkGreen
  0x006400
  @item DarkKhaki
  0xBDB76B
  @item DarkMagenta
  0x8B008B
  @item DarkOliveGreen
  0x556B2F
  @item Darkorange
  0xFF8C00
  @item DarkOrchid
  0x9932CC
  @item DarkRed
  0x8B0000
  @item DarkSalmon
  0xE9967A
  @item DarkSeaGreen
  0x8FBC8F
  @item DarkSlateBlue
  0x483D8B
  @item DarkSlateGray
  0x2F4F4F
  @item DarkTurquoise
  0x00CED1
  @item DarkViolet
  0x9400D3
  @item DeepPink
  0xFF1493
  @item DeepSkyBlue
  0x00BFFF
  @item DimGray
  0x696969
  @item DodgerBlue
  0x1E90FF
  @item FireBrick
  0xB22222
  @item FloralWhite
  0xFFFAF0
  @item ForestGreen
  0x228B22
  @item Fuchsia
  0xFF00FF
  @item Gainsboro
  0xDCDCDC
  @item GhostWhite
  0xF8F8FF
  @item Gold
  0xFFD700
  @item GoldenRod
  0xDAA520
  @item Gray
  0x808080
  @item Green
  0x008000
  @item GreenYellow
  0xADFF2F
  @item HoneyDew
  0xF0FFF0
  @item HotPink
  0xFF69B4
  @item IndianRed
  0xCD5C5C
  @item Indigo
  0x4B0082
  @item Ivory
  0xFFFFF0
  @item Khaki
  0xF0E68C
  @item Lavender
  0xE6E6FA
  @item LavenderBlush
  0xFFF0F5
  @item LawnGreen
  0x7CFC00
  @item LemonChiffon
  0xFFFACD
  @item LightBlue
  0xADD8E6
  @item LightCoral
  0xF08080
  @item LightCyan
  0xE0FFFF
  @item LightGoldenRodYellow
  0xFAFAD2
  @item LightGreen
  0x90EE90
  @item LightGrey
  0xD3D3D3
  @item LightPink
  0xFFB6C1
  @item LightSalmon
  0xFFA07A
  @item LightSeaGreen
  0x20B2AA
  @item LightSkyBlue
  0x87CEFA
  @item LightSlateGray
  0x778899
  @item LightSteelBlue
  0xB0C4DE
  @item LightYellow
  0xFFFFE0
  @item Lime
  0x00FF00
  @item LimeGreen
  0x32CD32
  @item Linen
  0xFAF0E6
  @item Magenta
  0xFF00FF
  @item Maroon
  0x800000
  @item MediumAquaMarine
  0x66CDAA
  @item MediumBlue
  0x0000CD
  @item MediumOrchid
  0xBA55D3
  @item MediumPurple
  0x9370D8
  @item MediumSeaGreen
  0x3CB371
  @item MediumSlateBlue
  0x7B68EE
  @item MediumSpringGreen
  0x00FA9A
  @item MediumTurquoise
  0x48D1CC
  @item MediumVioletRed
  0xC71585
  @item MidnightBlue
  0x191970
  @item MintCream
  0xF5FFFA
  @item MistyRose
  0xFFE4E1
  @item Moccasin
  0xFFE4B5
  @item NavajoWhite
  0xFFDEAD
  @item Navy
  0x000080
  @item OldLace
  0xFDF5E6
  @item Olive
  0x808000
  @item OliveDrab
  0x6B8E23
  @item Orange
  0xFFA500
  @item OrangeRed
  0xFF4500
  @item Orchid
  0xDA70D6
  @item PaleGoldenRod
  0xEEE8AA
  @item PaleGreen
  0x98FB98
  @item PaleTurquoise
  0xAFEEEE
  @item PaleVioletRed
  0xD87093
  @item PapayaWhip
  0xFFEFD5
  @item PeachPuff
  0xFFDAB9
  @item Peru
  0xCD853F
  @item Pink
  0xFFC0CB
  @item Plum
  0xDDA0DD
  @item PowderBlue
  0xB0E0E6
  @item Purple
  0x800080
  @item Red
  0xFF0000
  @item RosyBrown
  0xBC8F8F
  @item RoyalBlue
  0x4169E1
  @item SaddleBrown
  0x8B4513
  @item Salmon
  0xFA8072
  @item SandyBrown
  0xF4A460
  @item SeaGreen
  0x2E8B57
  @item SeaShell
  0xFFF5EE
  @item Sienna
  0xA0522D
  @item Silver
  0xC0C0C0
  @item SkyBlue
  0x87CEEB
  @item SlateBlue
  0x6A5ACD
  @item SlateGray
  0x708090
  @item Snow
  0xFFFAFA
  @item SpringGreen
  0x00FF7F
  @item SteelBlue
  0x4682B4
  @item Tan
  0xD2B48C
  @item Teal
  0x008080
  @item Thistle
  0xD8BFD8
  @item Tomato
  0xFF6347
  @item Turquoise
  0x40E0D0
  @item Violet
  0xEE82EE
  @item Wheat
  0xF5DEB3
  @item White
  0xFFFFFF
  @item WhiteSmoke
  0xF5F5F5
  @item Yellow
  0xFFFF00
  @item YellowGreen
  0x9ACD32
  @end table
  
  @anchor{channel layout syntax}
  @section Channel Layout
  
  A channel layout specifies the spatial disposition of the channels in
  a multi-channel audio stream. To specify a channel layout, FFmpeg
  makes use of a special syntax.
  
  Individual channels are identified by an id, as given by the table
  below:
  @table @samp
  @item FL
  front left
  @item FR
  front right
  @item FC
  front center
  @item LFE
  low frequency
  @item BL
  back left
  @item BR
  back right
  @item FLC
  front left-of-center
  @item FRC
  front right-of-center
  @item BC
  back center
  @item SL
  side left
  @item SR
  side right
  @item TC
  top center
  @item TFL
  top front left
  @item TFC
  top front center
  @item TFR
  top front right
  @item TBL
  top back left
  @item TBC
  top back center
  @item TBR
  top back right
  @item DL
  downmix left
  @item DR
  downmix right
  @item WL
  wide left
  @item WR
  wide right
  @item SDL
  surround direct left
  @item SDR
  surround direct right
  @item LFE2
  low frequency 2
  @end table
  
  Standard channel layout compositions can be specified by using the
  following identifiers:
  @table @samp
  @item mono
  FC
  @item stereo
  FL+FR
  @item 2.1
  FL+FR+LFE
  @item 3.0
  FL+FR+FC
  @item 3.0(back)
  FL+FR+BC
  @item 4.0
  FL+FR+FC+BC
  @item quad
  FL+FR+BL+BR
  @item quad(side)
  FL+FR+SL+SR
  @item 3.1
  FL+FR+FC+LFE
  @item 5.0
  FL+FR+FC+BL+BR
  @item 5.0(side)
  FL+FR+FC+SL+SR
  @item 4.1
  FL+FR+FC+LFE+BC
  @item 5.1
  FL+FR+FC+LFE+BL+BR
  @item 5.1(side)
  FL+FR+FC+LFE+SL+SR
  @item 6.0
  FL+FR+FC+BC+SL+SR
  @item 6.0(front)
  FL+FR+FLC+FRC+SL+SR
  @item hexagonal
  FL+FR+FC+BL+BR+BC
  @item 6.1
  FL+FR+FC+LFE+BC+SL+SR
  @item 6.1
  FL+FR+FC+LFE+BL+BR+BC
  @item 6.1(front)
  FL+FR+LFE+FLC+FRC+SL+SR
  @item 7.0
  FL+FR+FC+BL+BR+SL+SR
  @item 7.0(front)
  FL+FR+FC+FLC+FRC+SL+SR
  @item 7.1
  FL+FR+FC+LFE+BL+BR+SL+SR
  @item 7.1(wide)
  FL+FR+FC+LFE+BL+BR+FLC+FRC
  @item 7.1(wide-side)
  FL+FR+FC+LFE+FLC+FRC+SL+SR
  @item octagonal
  FL+FR+FC+BL+BR+BC+SL+SR
  @item downmix
  DL+DR
  @end table
  
  A custom channel layout can be specified as a sequence of terms, separated by
  '+' or '|'. Each term can be:
  @itemize
  @item
  the name of a standard channel layout (e.g. @samp{mono},
  @samp{stereo}, @samp{4.0}, @samp{quad}, @samp{5.0}, etc.)
  
  @item
  the name of a single channel (e.g. @samp{FL}, @samp{FR}, @samp{FC}, @samp{LFE}, etc.)
  
  @item
  a number of channels, in decimal, followed by 'c', yielding the default channel
  layout for that number of channels (see the function
  @code{av_get_default_channel_layout}). Note that not all channel counts have a
  default layout.
  
  @item
  a number of channels, in decimal, followed by 'C', yielding an unknown channel
  layout with the specified number of channels. Note that not all channel layout
  specification strings support unknown channel layouts.
  
  @item
  a channel layout mask, in hexadecimal starting with "0x" (see the
  @code{AV_CH_*} macros in @file{libavutil/channel_layout.h}.
  @end itemize
  
  Before libavutil version 53 the trailing character "c" to specify a number of
  channels was optional, but now it is required, while a channel layout mask can
  also be specified as a decimal number (if and only if not followed by "c" or "C").
  
  See also the function @code{av_get_channel_layout} defined in
  @file{libavutil/channel_layout.h}.
  @c man end SYNTAX
  
  @chapter Expression Evaluation
  @c man begin EXPRESSION EVALUATION
  
  When evaluating an arithmetic expression, FFmpeg uses an internal
  formula evaluator, implemented through the @file{libavutil/eval.h}
  interface.
  
  An expression may contain unary, binary operators, constants, and
  functions.
  
  Two expressions @var{expr1} and @var{expr2} can be combined to form
  another expression "@var{expr1};@var{expr2}".
  @var{expr1} and @var{expr2} are evaluated in turn, and the new
  expression evaluates to the value of @var{expr2}.
  
  The following binary operators are available: @code{+}, @code{-},
  @code{*}, @code{/}, @code{^}.
  
  The following unary operators are available: @code{+}, @code{-}.
  
  The following functions are available:
  @table @option
  @item abs(x)
  Compute absolute value of @var{x}.
  
  @item acos(x)
  Compute arccosine of @var{x}.
  
  @item asin(x)
  Compute arcsine of @var{x}.
  
  @item atan(x)
  Compute arctangent of @var{x}.
  
  @item atan2(x, y)
  Compute principal value of the arc tangent of @var{y}/@var{x}.
  
  @item between(x, min, max)
  Return 1 if @var{x} is greater than or equal to @var{min} and lesser than or
  equal to @var{max}, 0 otherwise.
  
  @item bitand(x, y)
  @item bitor(x, y)
  Compute bitwise and/or operation on @var{x} and @var{y}.
  
  The results of the evaluation of @var{x} and @var{y} are converted to
  integers before executing the bitwise operation.
  
  Note that both the conversion to integer and the conversion back to
  floating point can lose precision. Beware of unexpected results for
  large numbers (usually 2^53 and larger).
  
  @item ceil(expr)
  Round the value of expression @var{expr} upwards to the nearest
  integer. For example, "ceil(1.5)" is "2.0".
  
  @item clip(x, min, max)
  Return the value of @var{x} clipped between @var{min} and @var{max}.
  
  @item cos(x)
  Compute cosine of @var{x}.
  
  @item cosh(x)
  Compute hyperbolic cosine of @var{x}.
  
  @item eq(x, y)
  Return 1 if @var{x} and @var{y} are equivalent, 0 otherwise.
  
  @item exp(x)
  Compute exponential of @var{x} (with base @code{e}, the Euler's number).
  
  @item floor(expr)
  Round the value of expression @var{expr} downwards to the nearest
  integer. For example, "floor(-1.5)" is "-2.0".
  
  @item gauss(x)
  Compute Gauss function of @var{x}, corresponding to
  @code{exp(-x*x/2) / sqrt(2*PI)}.
  
  @item gcd(x, y)
  Return the greatest common divisor of @var{x} and @var{y}. If both @var{x} and
  @var{y} are 0 or either or both are less than zero then behavior is undefined.
  
  @item gt(x, y)
  Return 1 if @var{x} is greater than @var{y}, 0 otherwise.
  
  @item gte(x, y)
  Return 1 if @var{x} is greater than or equal to @var{y}, 0 otherwise.
  
  @item hypot(x, y)
  This function is similar to the C function with the same name; it returns
  "sqrt(@var{x}*@var{x} + @var{y}*@var{y})", the length of the hypotenuse of a
  right triangle with sides of length @var{x} and @var{y}, or the distance of the
  point (@var{x}, @var{y}) from the origin.
  
  @item if(x, y)
  Evaluate @var{x}, and if the result is non-zero return the result of
  the evaluation of @var{y}, return 0 otherwise.
  
  @item if(x, y, z)
  Evaluate @var{x}, and if the result is non-zero return the evaluation
  result of @var{y}, otherwise the evaluation result of @var{z}.
  
  @item ifnot(x, y)
  Evaluate @var{x}, and if the result is zero return the result of the
  evaluation of @var{y}, return 0 otherwise.
  
  @item ifnot(x, y, z)
  Evaluate @var{x}, and if the result is zero return the evaluation
  result of @var{y}, otherwise the evaluation result of @var{z}.
  
  @item isinf(x)
  Return 1.0 if @var{x} is +/-INFINITY, 0.0 otherwise.
  
  @item isnan(x)
  Return 1.0 if @var{x} is NAN, 0.0 otherwise.
  
  @item ld(var)
  Load the value of the internal variable with number
  @var{var}, which was previously stored with st(@var{var}, @var{expr}).
  The function returns the loaded value.
  
  @item lerp(x, y, z)
  Return linear interpolation between @var{x} and @var{y} by amount of @var{z}.
  
  @item log(x)
  Compute natural logarithm of @var{x}.
  
  @item lt(x, y)
  Return 1 if @var{x} is lesser than @var{y}, 0 otherwise.
  
  @item lte(x, y)
  Return 1 if @var{x} is lesser than or equal to @var{y}, 0 otherwise.
  
  @item max(x, y)
  Return the maximum between @var{x} and @var{y}.
  
  @item min(x, y)
  Return the minimum between @var{x} and @var{y}.
  
  @item mod(x, y)
  Compute the remainder of division of @var{x} by @var{y}.
  
  @item not(expr)
  Return 1.0 if @var{expr} is zero, 0.0 otherwise.
  
  @item pow(x, y)
  Compute the power of @var{x} elevated @var{y}, it is equivalent to
  "(@var{x})^(@var{y})".
  
  @item print(t)
  @item print(t, l)
  Print the value of expression @var{t} with loglevel @var{l}. If
  @var{l} is not specified then a default log level is used.
  Returns the value of the expression printed.
  
  Prints t with loglevel l
  
  @item random(x)
  Return a pseudo random value between 0.0 and 1.0. @var{x} is the index of the
  internal variable which will be used to save the seed/state.
  
  @item root(expr, max)
  Find an input value for which the function represented by @var{expr}
  with argument @var{ld(0)} is 0 in the interval 0..@var{max}.
  
  The expression in @var{expr} must denote a continuous function or the
  result is undefined.
  
  @var{ld(0)} is used to represent the function input value, which means
  that the given expression will be evaluated multiple times with
  various input values that the expression can access through
  @code{ld(0)}. When the expression evaluates to 0 then the
  corresponding input value will be returned.
  
  @item round(expr)
  Round the value of expression @var{expr} to the nearest integer. For example, "round(1.5)" is "2.0".
  
  @item sin(x)
  Compute sine of @var{x}.
  
  @item sinh(x)
  Compute hyperbolic sine of @var{x}.
  
  @item sqrt(expr)
  Compute the square root of @var{expr}. This is equivalent to
  "(@var{expr})^.5".
  
  @item squish(x)
  Compute expression @code{1/(1 + exp(4*x))}.
  
  @item st(var, expr)
  Store the value of the expression @var{expr} in an internal
  variable. @var{var} specifies the number of the variable where to
  store the value, and it is a value ranging from 0 to 9. The function
  returns the value stored in the internal variable.
  Note, Variables are currently not shared between expressions.
  
  @item tan(x)
  Compute tangent of @var{x}.
  
  @item tanh(x)
  Compute hyperbolic tangent of @var{x}.
  
  @item taylor(expr, x)
  @item taylor(expr, x, id)
  Evaluate a Taylor series at @var{x}, given an expression representing
  the @code{ld(id)}-th derivative of a function at 0.
  
  When the series does not converge the result is undefined.
  
  @var{ld(id)} is used to represent the derivative order in @var{expr},
  which means that the given expression will be evaluated multiple times
  with various input values that the expression can access through
  @code{ld(id)}. If @var{id} is not specified then 0 is assumed.
  
  Note, when you have the derivatives at y instead of 0,
  @code{taylor(expr, x-y)} can be used.
  
  @item time(0)
  Return the current (wallclock) time in seconds.
  
  @item trunc(expr)
  Round the value of expression @var{expr} towards zero to the nearest
  integer. For example, "trunc(-1.5)" is "-1.0".
  
  @item while(cond, expr)
  Evaluate expression @var{expr} while the expression @var{cond} is
  non-zero, and returns the value of the last @var{expr} evaluation, or
  NAN if @var{cond} was always false.
  @end table
  
  The following constants are available:
  @table @option
  @item PI
  area of the unit disc, approximately 3.14
  @item E
  exp(1) (Euler's number), approximately 2.718
  @item PHI
  golden ratio (1+sqrt(5))/2, approximately 1.618
  @end table
  
  Assuming that an expression is considered "true" if it has a non-zero
  value, note that:
  
  @code{*} works like AND
  
  @code{+} works like OR
  
  For example the construct:
  @example
  if (A AND B) then C
  @end example
  is equivalent to:
  @example
  if(A*B, C)
  @end example
  
  In your C code, you can extend the list of unary and binary functions,
  and define recognized constants, so that they are available for your
  expressions.
  
  The evaluator also recognizes the International System unit prefixes.
  If 'i' is appended after the prefix, binary prefixes are used, which
  are based on powers of 1024 instead of powers of 1000.
  The 'B' postfix multiplies the value by 8, and can be appended after a
  unit prefix or used alone. This allows using for example 'KB', 'MiB',
  'G' and 'B' as number postfix.
  
  The list of available International System prefixes follows, with
  indication of the corresponding powers of 10 and of 2.
  @table @option
  @item y
  10^-24 / 2^-80
  @item z
  10^-21 / 2^-70
  @item a
  10^-18 / 2^-60
  @item f
  10^-15 / 2^-50
  @item p
  10^-12 / 2^-40
  @item n
  10^-9 / 2^-30
  @item u
  10^-6 / 2^-20
  @item m
  10^-3 / 2^-10
  @item c
  10^-2
  @item d
  10^-1
  @item h
  10^2
  @item k
  10^3 / 2^10
  @item K
  10^3 / 2^10
  @item M
  10^6 / 2^20
  @item G
  10^9 / 2^30
  @item T
  10^12 / 2^40
  @item P
  10^15 / 2^40
  @item E
  10^18 / 2^50
  @item Z
  10^21 / 2^60
  @item Y
  10^24 / 2^70
  @end table
  
  @c man end EXPRESSION EVALUATION