OCL21 — Usage of initialization and derivation constraints on the same property

The OCL 2.0 spec seems to allow usage of initialization constraints
and derivation constraints on the same property. For example in
7.3.7, it says "Initial and derivation expressions may be mixed
together after one context.", which is a string indication that it is
not precluded. Having both initialization and derivation constraints
is an overspecification of the initial value of the property, since
the derivation constraint must be satisfied at any time, which
probably includes the initialization time.

Also, the spec does not seem to contain a statement about how many
initialization and derivation constraints are allowed on a property.
By the nature of these constraints, it seems sensible to have at most
one of them.

The following clarifications are suggested to address this issue:
(1) clarify whether "at any time" for derivation constraints
includes the initialization. Suggestion: Derivation should include
initialization.
(2) clarify whether both kinds of constraints are allowed on the
same property. Suggestion: Both are allowed but must not be
contradictory.
(3) clarify how many initialization and derivation constraints are
allowed on a property. Suggestion: At most one initialization
constraint and at most one derivation constraint.

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