[metapost] MP docs

[Stephan Hennig]

Dan Luecking schrieb:

> When first discussing cmykcolor (section 5.1) mpman says black would
> be expressed as (1,1,1,1). Perhaps (0,0,0,1) would be better?

Obviously, the best way to refer to black in cmyk colour space were to
say 'black'.  Currently, pre-defined constants are missing for all basic
cmyk colours.  Since 'black' and 'white' are already defined as
rgbcolors I propose adding the following cmykcolor constants

cyan       = (1,0,0,0)
magenta    = (0,1,0,0)
yellow     = (0,0,1,0)
cmykblack  = <Whatever experts think about this.>
cmykwhite  = (0,0,0,0)

The user-friendly solution were to have 'black' and 'white' behave as an
rgbcolor /or/ cmykcolor depending on the context, but I don't know if
that is possible in MetaPost.

Best regards,
Stephan Hennig