:: deftheorem Def4 defines angle COMPLEX2:def 4 :
for x, y, z being Complex holds
( ( (Arg (z - y)) - (Arg (x - y)) >= 0 implies angle (x,y,z) = (Arg (z - y)) - (Arg (x - y)) ) & ( not (Arg (z - y)) - (Arg (x - y)) >= 0 implies angle (x,y,z) = (2 * PI) + ((Arg (z - y)) - (Arg (x - y))) ) );