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Pack julian -- prolog/julian/calendar/gregorian.pl
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 gregorian(?Year, ?Month, ?Day) is semidet
Constrain Year, Month and Day to a valid date in the Gregorian calendar. For example, one could iterate all leap years since 1950 with this:
gregorian(Y, 2, 29), Y #> 1950, indomain(Y).

Because it just constrains Year, Month and Day to have the proper relation one to another, one can bind as many or as few of the arguments as desired.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 month_number(Arg1, Arg2)