theorem Th57:
  x1 <> x2 & x1 <> x3 & x2 <> x3 implies card {x1,x2,x3} = 3
proof
  assume x1 <> x2 & x1 <> x3 & x2 <> x3;
  then
A1: card {x1,x2} = 2 & not x3 in {x1,x2} by Th56,TARSKI:def 2;
  {x1,x2,x3} = {x1,x2} \/ {x3} by ENUMSET1:3;
  hence card {x1,x2,x3} = 2+1 by A1,Th40
    .= 3;
end;
