If it's ArrayLiteralExp, its elements will be copied. Otherwise, e1 itself will be pushed into the new Expressions.
If it's not null, it will be pushed/appended to the new Expressions by the same way with e1.
Newly allocated `Expresions. Note that it points the original Expression values in e1 and e2.
Copy element Expressions in the parameters when they're ArrayLiteralExps.