def p_length_unit(units):
   """Returns length units string as expected by pagoda weights and measures module."""

   # NB: other length units are supported by resqml
   if units.lower() in ['m', 'metre', 'metres']: return 'metres'
   if units.lower() in ['ft', 'foot', 'feet', 'ft[us]']: return 'feet'
   assert(False)