atan
_DEFINE_EXPR_UNARY_FUNCTION(atan)
MAPM_1aryFunc(atan)
MAPM_1aryFunc(atan)
extern double atan(double x);
float angle = rad2deg(atan(dy / dx));
return _ParseFactorial(values[0].atan());
return atan(y);
z = atan(fabs(y/x));
return atan(x);