return (Expression)primitiveItems.get(exp);
else {
// if this is the first time, wrap it and memorize it.
Datatype dt = exp.getType();
PrimitiveItem p = new PrimitiveItem(
(dt instanceof DatabindableDatatype)?(DatabindableDatatype)dt:null,
(Expression)exp);
primitiveItems.put( exp, p );
return p;
}