Prolog beginner, member/2 builtin predicate -

i have this:

someuserdef(a,[b,c,d]). 

when try

?- someuserdef(a,l),member(b,l). 

i don't true or false answer list

l=[b,c,d] ; false. 

how true or false answer?

you're asking prolog give value of l matches both rules. if can tell there's 1 answer, return yes - otherwise give first answer, return no, , prompt before checking more answers.

when you're providing variables (capital letters) prolog queries, return matches - it's when ask check rules (with no variables) e.g. someuserdef(b, [b, c, d]) return yes or no indicate whether assertion true or false.