
theorem Th2:
  for X be non empty set, f be PartFunc of X,ExtREAL holds -(-f) = f
proof
   let X be non empty set, f be PartFunc of X,ExtREAL;
A1:dom f = dom (-f) by MESFUNC1:def 7; then
A2:dom f = dom (-(-f)) by MESFUNC1:def 7;
   now let x be object;
    assume A3: x in dom f; then
    (-f).x = -(f.x) by A1,MESFUNC1:def 7; then
    (-(-f)).x = -(-(f.x)) by A2,A3,MESFUNC1:def 7;
    hence (-(-f)).x = f.x;
   end;
   hence thesis by A2,FUNCT_1:2;
end;
