INTERNAL:
Used by function operators in deployment xml generation to accomodate custom functions.
There is no more validation on read because any custom function has to be accepted.
The custom function is assumed to be a normal prefix function.
INTERNAL:
Represents the aliased tables an ObjectExpression will be translated to,
along with any of its derived TableExpressions.
For bug 2778339 / CR 2456, this Lookup also represents identity.