Undertaking ( in one section of his book " Past, Present, and Future " ( 1967 ) ) the attempt to combine binary ( e . g ., " until " ) and unary ( e . g ., " will always be " ) temporal operators to one system of temporal logic, Prior as an incidental result builds a base for later hybrid languages.
12.
These formulae are called " state formulae ", while those created by the symbol \ phi are called " path formulae " . ( The above grammar contains some redundancies; for example \ Phi \ or \ Phi as well as implication and equivalence can be defined as just for Boolean algebras ( or propositional logic ) from negation and conjunction, and the temporal operators "'X "'and "'U "'are sufficient to define the other two .)
