List of kids of the expression.
Set of atoms found in the cone of influence of the expression.
Height of the expression, by convention literals have height 0.
Set of literals found in the cone of influence of the expression.
Number of kids.
True iff the atom sets of kids are mutually disjoint, ie iff they are syntactically independent, and hence semantically independent.
True iff the expression has no duplicate kid.
Abstract base type for all NNF expressions.