Question: How to define a boolean function?

I´d like to use the remove command, to select elements of a list.

E.g.

L:=[1,2,3,4,5,6,7,3,9];

L1:=remove('boolean function',L);

Where can I get an overwiew of all boolean functions, i.e. commands, that output true, false or fail?

E.g. I wanted to simply say

L1:=remove(<>3,L);

which should just leave all "3" values in L1, but that didn´t work. So I needed to define a boolean function to say "if element of List L is not 3, then remove it"...

Where can I view all permitted commands for that first argument of remove, or select?

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