def _evaluate_usecols(usecols, names):
if callable(usecols):<EOL><INDENT>return {i for i, name in enumerate(names) if usecols(name)}<EOL><DEDENT>return usecols<EOL>
Check whether or not the 'usecols' parameter
is a callable.  If so, enumerates the 'names'
parameter and returns a set of indices for
each entry in 'names' that evaluates to True.
If not a callable, returns 'usecols'.