se, return 'None'.

    Return value is based on the first assignment to 'symbol'.  'symbol' must
    be a global, or at least a non-"fast" local in the code block.  That is,
    only 'STORE_NAME' and 'STORE_GLOBAL' opcodes are checked, and 'symbol'
    must be present in 'code.co_names'.
    NéZ