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

theorem Th19:
  x - y = -infty implies (x = -infty or y = +infty)
proof
  assume
A1: x - y = -infty;
  assume ( not x = -infty)& not y = +infty;
  then x in REAL & y in REAL or x in REAL & y = -infty or x = +infty & y in
  REAL or x = +infty & y = -infty by XXREAL_0:14;
  hence thesis by A1,Th13,Th14;
end;
