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Question: How to extract the names of the functions?

Question:How to extract the names of the functions?

mmcdara 6149 Maple
indets
June 05 2020
1

Hi, 

I have an expression like this one 
x := -h(U) + f( f( g( f(U+W*X)*V) + g( f(W)*g(V) ) ) );
I and I would like to extract the names f, g, h of the functions it contains.
I'v been stucked on this problem for a while and I finally found this could be done doing this 
op~(0, indets(x, function({name, `+`, `*`})))

I don't understand why this works not even if it is the good way to proceed (the few examples I've built seem to prove this command works).
What way would you use to tackle this problem?

TIA
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