theorem
  (ex v st v is isolated) & G is c-Dregular implies c = 0
proof
  assume A1: (ex v st v is isolated) & G is c-Dregular;
  then consider v such that
    A2: v is isolated;
  thus c = v.inDegree() by A1
    .= 0 by A2, GLIB_000:156;
end;
