theorem Th38:
  arccot (-1) = 3/4*PI & arccot.(-1) = 3/4*PI
proof
A1: 3/4*PI < PI by Lm10,XXREAL_1:4;
  arccot (-1) = 3/4*PI by A1,Th18,Th36;
  hence thesis;
end;
