[R6RS] "shall"

[Michael Sperber]

I've added a more precise explanation of the use of "shall" to
terminology.tex and replaced "must" by "shall" in the places where
it's appropriate.  ("Shall" is used to express restrictions on
procedure arguments or subforms of syntax, where non-conformance
causes an exception to be raised.)  Please follow that terminology for
new text.

No doubt I haven't done this as thoroughly as possible.

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla