Replaces NaNs with zero. A NaN (Not a Number) is a floating point data value which represents an undefined or unrepresentable value, such as the result of dividing by zero. Computations on NaNs produce more NaNs, and so it is often preferable to replace the NaN with a zero value. Note that division and modulo operators in gen~ protect against generating NaNs by default, so this should only be required for protecting against incoming values in exported code.