Euler Method Recursive Calc for HP Prime

#cas
euler_it(f,k,z,step_size,goal):=
BEGIN
z := z+step_size*subst(f,[x,y],[k,z]);
k := k+step_size;
IF k =goal THEN
 return z;
ELSE
 euler_it(f,k,z,step_size,goal);
END;
END;
#end