theorem Th7: :: left-self-distributivity
  a@b = a implies a = b
proof
  assume
A1: a@b = a;
  consider x such that
A2: x@a = b by Def3;
  b = (x@a)@b by A2,Def3
    .= (x@b)@a by A1,Th5
    .= a by A1,A2,Th5;
  hence thesis;
end;
