= Thoughts on references =

Create interfaces:
* `ReferenceExpression extends SymbolicExpression`
* `NTReferenceExpression extends ReferenceExpression` for non-trivial references, i.e., references to a tuple component, array element, or union member, or an "offset reference"
* `ArrayElementReference extends NTReferenceExpression`
* `TupleComponentReference extends NTReferenceExpression`
* `UnionMemberReference extends NTReferenceExpression`
* `OffsetReference extends NTReferenceExpression`

Interface:

The following methods can go in `ReferenceExpression`:

{{{
/** Is this the null reference? */
boolean isNullReference();

/** Is this the identity reference? */
boolean isIdentityReference();

/** Is this an array element reference? */
boolean isArrayElementReference();

/** Is this a tuple component reference? */
boolean isTupleComponentReference();

/** Is this a union element reference? */
boolean isUnionMemberReference();

/** Is this an "offset reference" ? */
boolean isOffsetReference();
}}}

The following methods can go in `NTReferenceExpression`:

{{{
/** As this is a reference to an array element, tuple component, union member, 
 * or an offset reference, returns the reference to the parent,
 * i.e., the array, tuple, union, or the  other reference, resp.
ReferenceExpression getParent();
}}}

In `ArrayElementReference`:
{{{
NumericExpression getIndex();
}}}

In `TupleComponentReference`:
{{{
IntObject getIndex();
}}}

In `UnionMemberReference`:
{{{
IntObject getIndex();
}}}

In `OffsetReference`:
{{{
NumericExpression getOffset();
}}}


The following can go in `SymbolicUniverse`:

{{{
/** Returns the type of all reference expressions. */
SymbolicType referenceType();

/** Returns the "null reference", a symbolic expression of reference type which is
 * not equal to a reference value returned by
 * any of the other methods, and which cannot be dereferenced. */
ReferenceExpression nullReference();

/** Given a reference and a value, returns the sub-expression of value specified by the reference.
 * Throws exception if the reference is not of the correct form for the type of value. */
SymbolicExpression dereference(SymbolicExpression value, ReferenceExpression reference);

/** Returns the identity (or "trivial") reference I.  This is the reference characterized by the
 * property that dereference(I,v) returns v for any symbolic expression v. */
ReferenceExpression identityReference();

/** Given a reference to an array and an index (integer), returns a reference to the element of the array
 * at that index */
ArrayElementReference arrayElementReference(ReferenceExpression arrayReference, NumericExpression index);

/** Given a reference to a tuple, and a field index, returns a reference to that component of the tuple */
TupleComponentReference tupleComponentReference(ReferenceExpression tupleReference, IntObject fieldIndex);

/** Given a reference to a union (expression of union type) and an index of  a member type of that union, returns
 * a reference to the underlying element */
UnionMemberReference unionElementReference(ReferenceExpression unionReference, IntObject memberIndex);

OffsetReference offsetReference(ReferenceExpression reference, NumericExpression offset);

/** Given a symbolic expression value, a reference to a point within that value, and a subValue, returns
 * the symbolic expression obtained by replacing the referenced part of value with subValue. */
SymbolicExpression assign(SymbolicExpression value, ReferenceExpression reference, SymbolicExpression subValue);
}}}