a picadillo

[M Douglas McIlroy]

The pic code below intends to specify a tailless arrow. (Linewid, not so
intuitively, sets the default length of horizontal line segments.)
.PS
linewid  = 0
arrow right
.PE
Pic balks: "cannot draw arrow on object with zero length". Yet a point and
a direction are all one needs to place an arrowhead. (The current point in
the example is (0,0).)
A workaround is to make linewid smaller than arrowht (the length of an
arrowhead) but not zero. Nevertheless, the example violates Kernighan's law
about handling zeros, "Do nothing gracefully."