'Returns the label that matches the position regardless of the
value.
For positions ``pos < len(seq)``, return `seq[i]` regardless of
`x`. Otherwise return empty string. `seq` is the sequence of
strings that this object was initialized with.'

def __call__(self, x, pos=None):

    if ((pos is None) or (pos >= len(self.seq))):
        return u''
    else:
        return self.seq[pos]
