'Returns the field with the provided field name.
Args:
field_name: The name of the field to return.
Returns:
A field with the given name.
Raises:
ValueError: There is not exactly one field with the given name.'

def field(self, field_name):

    fields = self[field_name]
    if (len(fields) == 1):
        return fields[0]
    raise ValueError(('Must    have    exactly    one    field    with    name    %s,    but    found    %d.' % (field_name, len(fields))))
