It is possible to interactively experiment the effect of figure parameters. This requires: 1) MetaPost code written as "anglinerigorouscircle.mp". 2) xcmd package (kindly provided by Pedro Sebastião); it may be downloaded from http://lince.cii.fc.ul.pt/ in Debian, Red Hat or tar.gz package formats. 3) Perl. 4) gv. 5) run command "xcmd/xmpost anglinerigorouscircle" or "xcmd/xmpost anymetapostwoextension".