def sub(a, b):
if a is None:<EOL><INDENT>if b is None:<EOL><INDENT>return None<EOL><DEDENT>else:<EOL><INDENT>return -<NUM_LIT:1> * b<EOL><DEDENT><DEDENT>elif b is None:<EOL><INDENT>return a<EOL><DEDENT>return a - b<EOL>
Subtract two values, ignoring None