The function type(expr, 'assignable') returns true if the expression expr can be assigned to; otherwise, it returns false.

If the parameter t is included, it checks that expr is also of that type.

An expression is assignable if one of the following is true:

- It is a symbol and it is not protected.

- It is of type indexed and its zeroth operand is either assignable or a table or an rtable.

- It is of type function and its zeroth operand is assignable.