Thanks for the nice bug report. For what it's worth, mocl was naively dividing the arguments per "If both number1 and number2 are supplied for atan, the result is the arc tangent of number1/number2". But later in the spec you can see "For atan, the signs of number1 (indicated as x) and number2 (indicated as y) are used to derive quadrant information". This wasn't happening.
Fixed in 21967b0.