substitution in expression

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Hi,

After reading Simplify shenanigans, I am very confused about the possibility of doing

sage: sin(x).subs(sin(x) == 3)
3

Which becomes even more funny with

sage: SR(2).subs(SR(2) == pi)
pi

I thought one may only set values to variable but no: I can set value to integers !

Is there a concrete example where this is useful ? Or this is just one more argument to avoid the use of the symbolic ring ?

Anyway the substitution is not at all consistent with the behavior of the symbolic ring. If such substitutions are allowed, then the expression SR(2) + SR(2) should not be simplified !