# abs(x) is abolute value function;
# abs(inverseabs(x_)):=x;
# abs(zero) := zero;
# abs(-zero) := -zero;
# abs(x_,y_):=hypot(x,y)=sqrt(x^2+y^2);
# abs(x_) :=  If(x>0, x, If(x<0,-x));

abs(a_*b_):= abs(a)*abs(b);
abs(point(x_,y_)):=abs(x,y);
abs(vector(x_,y_)):=abs(x,y);
abs(vector(x_,y_,z_)):=abs(x,y,z);