MID$() can not be used on the left-hand-side of an assignment

Description

In other BASIC dialects it is possible to assign a value to the MID$() function to replace a portion of a string.

Example code to demonstrate the bug.

    String$ = "The quick brown fox jumps over the lazy dog."
    MID$(String$, 11, 5) = "red"

Example code to work around the bug.

' build a custom replace$() function