cos(1,x):=cos(x); cos(-x_):=cos(x); cos(acos(x_)):=x; cos(pi) := (-1); cos(2pi):=1; cos(0.25*pi):=sqrt(2)/2; cos(2pi+x_):=cos(x); cos(0.5pi+x_):= -sin(x); cos(-0.5pi+x_):=sin(x); cos(-2pi+x_):=cos(x); cos(pi+x_):= -cos(x); cos(-pi+x_):= -cos(x);