theorem Th4:
  (SCM*-VAL*SCM+FSA-OK).NAT = NAT
proof
A1: SCM+FSA-OK.NAT = 0 by Lm4;
  thus (SCM*-VAL*SCM+FSA-OK).NAT
     = SCM*-VAL.(SCM+FSA-OK.NAT) by Lm3,FUNCT_1:13
    .= NAT by A1;
end;
