reserve x,y,z,w for ExtReal,
  r for Real;
reserve f,g for ExtReal;
reserve t for ExtReal;

theorem Th93:
  y = -z implies x*(y+z) = x*y +x*z
proof
  assume
A1: y = -z;
  hence x*(y+z) = x*0 by Th7
    .= x*y - x*y by Th7
    .= x*y + x*(-y) by Th92
    .= x*y + x*z by A1;
end;
