This method determines if two elements are comparable and further if a is less than or equal to b in P.
i1 : P = poset {{a,b},{a,c}};
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i2 : compare(P, a, b)
o2 = true
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i3 : compare(P, c, a)
o3 = false
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If two elements are incomparable, then the result is false.
i4 : compare(P, b, c)
o4 = false
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