Burkhard,
You're quite right.  I spoke (emailed) too quickly when I said "supposed
to" might be a single morpheme.  It has at least a couple.  But I think the
concept of "quasi-modals" is a legitimate category even if--like all other
categories of grammar--not leak-proof.  A quasi-modal is (to take a stab at
an informal definition) a group of words, normally two, that function as a
auxillary verb.  Furthermore, the definition must hinge on this: the two
words semantically as well as grammatically work together & breaking them
apart would be counter-intuitive.  Sometimes, then, this name is applied by
default--when, as with the sentence in question here--there seems to be no
other way to analyze the sentence.
 
Larry Beason,Director
English Composition Program
Dept. of English
Eastern Washington University
Cheney WA 99004
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