A step on the @R05174@ of some @S05970@ in which the @M03986@ C is already present in an @E02090@ or @E02087@ with A during the formation of B from A, e.g. In this mechanism the @CT01038@ C may but does not necessarily assist the formation of B from A, which may itself be a @M03989@ reaction with some other @R05190@. Pre-@A00472@ is important when B is too short-lived to permit B and C to come together by @D01716@.