[boost] [concept] why is `requires` plural?

Hello all, This might be a very silly curiosity: Does anyone know why `requires` (plural) is used to specify concept requirements? Most keywords (all?) are not plural so I would have expexted `require` to be used instead of `requires`. Howerver, the C++0x proposals and ConceptC++ use `requires`. Consequently, Boost.ConceptCheck uses BOOST_CONCEPT_REQUIRES(). Do you know why the plural form was selected? Thank you. -- Sent from my mobile device Lorenzo