mfpic-doc.pdf.

4.4 MODIFYING THE FIGURES. 30
\lclosed\arc{(0,0),(1,0),45}.
These are always prefixed to some figure drawing command, and apply only to the next following
figure macro provided that only other prefix commands intervene. This is a rather long section, but
even more modification prefixes are documented in subsection 4.10.2.
The combination of a modifying macro, followed by a figure macro, can usually be thought of
as a new figure macro, to which further prefixes might be prepended.
More precisely: all prefix macros have an input path, an output path, and a side effect. The input
is the path that is output by the following prefix or figure macro. The output is either the same as the
input or a modification of it. The side effect might be a drawing or filling of the path or the addition
of an arrowhead.
We list here a classifications of prefix and figure macros that is useful for understanding the
MFPIC system.
Figure macros. These take no input path; they must come last in a sequence. They output the path
they were designed to produce. Examples are \circle, \rect and \polygon. If they have no
prefixes, or are preceded only by appending macros (see next), they invoke a default rendering
of the path (usually a drawing as a solid stroke) as the side effect.
Appending macros These pass their input unchanged as their output. Their side effect is the appending
of some object such as an arrow head or tail. Currently only the various prefix macros
whose names begin with arrow are appending macros (see subsection 4.4.3). But \reverse,
which technically modifies a path and has no side effect, is coded as an appending macro so
that it will work correctly with arrows. Think of it as ‘appending’ a new direction.
Rendering macros These pass their input unchanged as their output. They have the side effect of
adding or subtracting ink from a picture in the shape of the input path. Examples are \draw,
\dotted, \gfill and \gclip.
Modifying macros These output the result of applying their intended modification to the input path.
Examples are macros that close the path if it was open, macros that apply a transformation
such as a rotation, and macros that return only a part of a path. If they have no prefixes, or are
preceded only by appending macros (see above), they also invoke a default rendering of the
output path (usually a drawing as a solid stroke of the modified path) as the side effect.
4.4.1 CLOSURE OF PATHS
It should be pointed out that the closure macros will leave already closed paths unchanged, so it
is always safe to add one when uncertain. Moreover, if the path is not closed but the endpoints are
identical, \lclosed and \bclosed will close it without adding any path segment.
\lclosed...
\bclosed[〈tens〉]...
\sclosed[〈tens〉]...
These modifying macros all turn an open path into a closed one. If the path is already closed,
they do nothing.
\lclosed makes an open path into a closed path by adding a line segment between the endpoints
of the path. In the special case where the path ends exactly where it begins, all \lclosed does is
change the type of the path from open to closed.
The \bclosed macro is similar to \lclosed, except that it closes an open path smoothly by
drawing a Bézier curve. A Bézier is METAFONT’s natural way of connecting points into a curve, and
\bclosed is the simplest and most efficient closure next to \lclosed. Moreover it usually gives a
reasonably aesthetic result. Sometimes, however, one might wish a tighter connection. If that is the