MetaFun - Pragma ADE

Since we don't consider this unfill a suitable operator, you may wonder how we achieved the
above result.
fill (0,0)--(0,h)--(w,h)--(w,0)--(ww,0)--(w,hh)--(ww,hh)--
(ww,0)--cycle ;
This feature depends on the POSTSCRIPT way of filling closed paths, which comes down to filling
either the left or the right hand side of a curve. The following alternative works too.
fill (0,0)--(0,h)--(w,h)--(w,hh)--(ww,hh)--(ww,0)--(w,hh)--
(w,0)--cycle ;
The next alternative will fail. This has to do with the change in direction at point (0,0) halfway
through the path. Sometimes changing direction can give curious but desirable effects, but here it
brings no good.
fill (0,0)--(0,h)--(w,h)--(w,0)--(0,0)--(ww,0)--(ww,hh)--
(w,hh)--(ww,0)--cycle ;
This path fails because of the way POSTSCRIPT implements its fill operator. More details on how
POSTSCRIPT defines fills can be found in the reference manuals.
Some of the operations we have seen are hard coded into METAPOST and are called primitives.
Others are defined as macros, that is, a sequence of METAPOST commands. Since they are used
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