In Prolog why does this query return this particular result

?- assert(p(a)),assert(p(b)),p(X).
X = a 
yes

Wha开发者_如何学运维ts the effect of this query and why does it return this particular result?

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It is as if you queried the following source:

input.pl:

p(a).
p(b).

?- p(X)

X = a
yes