troff/font/devutf/charlib/RC

/build_RC {
    pop
    size 4 div /side exch def
    currentpoint
    newpath
    moveto
    0 side 1.5 mul rmoveto
    0 side rlineto
    side 2.5 mul 0 rlineto
    0 side neg rlineto
    closepath
    fill
} def