Redundant assumptions

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I want to verify certain inequalities involving real numbers and infinity. One case is showing certain expression is less than infinity. However, the following situation really puzzles me:

a = SR.var('a')
bool(a < infinity)

It returns 'False'. But when I do

assume(a < infinity)

It says

ValueError: Assumption is redundant

Why is this the case and how should I get 'True' for 'bool(a < infinity)'?

Thanks!