'Get the named field object (ambiguity is an error).'

def __getitem__(self, name):

    fields = self.fields.get(name)
    assert (fields is not None), ('No    field    by    the    name    %r    found' % name)
    assert (len(fields) == 1), ('Multiple    fields    match    %r:    %s' % (name, ',    '.join(map(repr, fields))))
    return fields[0]
