1) Declare two local variables:
   clock c;
   int b;
2) Add a self-loop transition to the initial state, with the guard:
   b>3 || c<2
3) Run a syntax check: passes.
4) Change the guard to:
   c<2 || b>3
5) Run a syntax check: fails with: Expression of type (constraint) cannot be
used as a guard, even though the two guards are logically equivalent and
syntactically similar.

The code is quite scattered but formally the typechecker seems to implement the
following rules:

invariant ::= integral | (clock "<" clock) | (clock ">" clock) 
          | (clock "<" integer) | (integer ">" clock) 
          | (difference "<" integer) | (integer "<" difference) 
          | (difference ">" integer) | (integer ">" difference)
          | (invariant "&&" invariant) | (integral "||" invariant)
          ;

guard ::= invariant | (integer "<" clock) | (clock ">" integer) 
          | (guard "&&" guard) | (integral "||" guard)
          ;

constraint ::= guard | (clock "!=" clock) | (clock "!=" integer) 
          | (integer "!=" clock) | (difference "!=" clock)
          | (difference "!=" clock) | "!" constraint | "forall" constraint
          | "exists" constraint | (constraint "&&" constraint) 
          | (constraint "||" constraint)           
          ;

Typechecker will try to interpret the expression in simplest possible way (i.e.
expression will be invariant if it matches invariant rule, and expression will
get promoted to guard only if it cannot match invariant anymore and so on). 

Eventually some expressions might end up as constraints in general, which means
 that they are unsupported, they are easily recognized (type CONSTRAINT) and
raise "cannot be used as" errors.

As you can see from the rules, the reported problem persists also for
invariants.

Having said that, the following additional rules enable typechecker to accept
the above expressions:

invariant ::= | (invariant "||" integral)

guard ::= | (guard "||" integral)

This fix has been applied in rev. 3546.
Behold the rest of the code :-)

Thanks Marius. I also appreciate your inclusion of the rules.