theorem
  dom f misses dom h & f c= g +* h implies f c= g
proof assume that
A1: dom f misses dom h and
A2: f c= g +* h;
A3: (g +* h)|dom f = g|dom f +* h|dom f by Th71
      .= g|dom f +* {} by A1,RELAT_1:66
      .= g|dom f;
  f|dom f = f;
  then f c= g|dom f by A2,A3,RELAT_1:76;
 hence f c= g by RELAT_1:184;
end;
