Filters the executables in the dictionary by their type.

def _filter_execs(self, isSubroutine):
        """Filters the executables in the dictionary by their type."""
        result = {}
        for key in self.executables:
            if (isinstance(self.executables[key], Subroutine) and isSubroutine) or \
               (isinstance(self.executables[key], Function) and not isSubroutine):
                result[key] = self.executables[key]
        
        return result