> The selectors in parentheses may be replaced with other selectors by modifying the compiler and recompiling all methods in the system. The other selectors are built into the virtual machine.

> Any objects referred to in a CompiledMethod's bytecodes that do not fall into one of the categories above must appear in its literal frame. The objects ordinarily contained in a literal frame are

> shared variables (global, class, and pool)

> most literal constants (numbers, characters, strings, arrays, and symbols)

> most message selectors (those that are not special)

> Objects of these three types may be intermixed in the literal frame. If an object in the literal frame is referenced twice in the same method, it need only appear in the literal frame once. The two bytecodes that refer to the object will refer to the same location in the literal frame.

> Two types of object that were referred to above, temporary variables and shared variables, have not been used in the example methods. The following example method for Rectangle merge: uses both types. The merge: message is used to find a Rectangle that includes the areas in both the receiver and the argument.

http://www.mirandabanda.org/bluebook/bluebook_chapter26.html

Evaluating this yields an error in both Squeak and Pharo. What Smalltalk are you using? I'm going to guess Cuis, in which case your example holds, but is misleading. Consider:

    a:='justastring'.
    b:='justastring'.
    a at: 6 put: $S.
    a, ' = ', b.
    'justaString = justaString' .