Corresponds to the grammar rule atomic_sentence.
Holds the left and right operands in the rules conjunction, disjunction,
exclusive_disjunction, implication, and biconditional.
Corresponds to the grammar rule constraint.
Corresponds to the grammar rule informal_constraint.
Corresponds to the grammar rule controlled_language_tag.
Corresponds to the grammar rule cardinality.
Corresponds to the grammar rule function_composition.
Corresponds to the grammar rule functional_term.
Corresponds to the grammar rule quantified_sentence.
Corresponds to the grammar rule quantified_variable.
Corresponds to the grammar rule sequence_comprehension.
Holds the left and right operands in the rules conjunction, disjunction,
exclusive_disjunction, implication, and biconditional.