Question: Detect if a input is changed WITHOUT re-evaluating it?

For instance, here is a worksheet where I have replaced 1 + 2; with 1 + 1; in advance: 

1+1 = 21+1 = 3

1 + 1;



1 + 1;




But without re-evaluations, how do I know the second input and the fourth input have been stealthily substituted?

In a Mma notebook, this can be displayed by shadowing the input label: 

But in order to detect changes, which method should be used in Maple?
(Note that I do not mean that no changes are allowed. So I don't think setting the “editable” property is a workaround.)

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