(< @X 18) doesn't behave as expected with pilog

[CILz]

Hello,

Let's say that I have those two facts in a pilog database:
(be age (Paul 18))
(be age (Vincent 17))

I'm looking for the guy under 18 with this rule:

(be underage (@X)
  (age @X @Y)
  (< @Y 18))

If I ask (? (underage @X)) the result here is -> NIL where I expect to 
get @X=Vincent.

If I modify the above rule with:

(be underage (@X)
  (age @X @Y)
  (equal @Y 18))

The same query (? (underage @X)) now gives @X=Paul which is the expected 
result.

I'm sure I'm missing something in the first case but I don't know what. 
Any idea?

Thanks,

Eric

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