cubicweb #509108 reflexivity of relationships [waiting feedback]

if a relation R is reflexive, it can link an object to itself. For example, same_value_as is reflexive, but child_of is not, event though both relations can link two objects of the same type A.

Non-reflexivity can currently be expressed with a constraint, but passing a parameter to the constructor or setting an attribute to True/False would be better.

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