Does `side-effect-free` have to apply to sub-functions too?

Hello,

I am wondering whether a function being free of side effects applies to 
functions that use `seq-elt`, which could run an unknown method 
implementation for an unknown custom type. If it is possible for the 
unknown method implementation to have side effects, then is the function 
which unknowingly triggers the unknown implementation considered to not 
be side-effect free?

In other words, before marking a function as `side-effect-free`, must 
one check the function's sub-functions, and those sub-functions' 
sub-sub-functions, and so on, all the way down, even if the function 
itself does not intentionally have a side effect?

Thank you.