groff/pic assertion fails

[Doug McIlroy]

With groff/pic version 1.18.1.1 (redhat fedora) and 1.18.1 (cygwin)
the following Bourne shell script produces the diagnostic
        assert failed *minx > *maxx
It's a sensitive problem. Changing 1.2247 to 1.224 or 1.225
makes it go away, as does changing .PS 3.5i to .PS 4i or
140*radians to 3*atan2(1,1).  Then it gives the expected picture.

groff -p >/dev/null <<'!'
.PS 3.5i
radians = 3.14159265/180
define polar {(cos($1),sin($1))}
C1: "" at (0,-.707)
C3: "" at (-1.2247,0)
arc rad 1  from C1+polar(140*radians) to C1+polar(180*radians)
.PE
!