MM3 force field terms/interrelationship

Me again...
 Looking at the out-of-plane term, it seems that a single bond angle
 is split for planar-center atoms into an in-plane and out-of-plane
 component.  This is relatively straightforward...
 In MM3, there are bend-bend and stretch-bend terms - when planar-
 center atoms are present, are the "original" angle values used, or
 are the in-plane terms substituted in the b-b and s-b terms?  I've
 looked at the MM2 I've worked with over the years, and the code's
 not clear (or commented, really) - and this kind of stuff isn't well
 covered in the literature (for either MM2 or MM3...)
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