1.Previous
3.Next

can be defined recursively as follows.

#If is an atomic formula, then is a formula of .
#If is a formula of , then is too.
#If and are formulas of , then is too.
#If is a formula of , then is too.
#If is a formula of , then is too.

Modal operators can be added to other kinds of logic by introducing rules analogous to #4 and #5 above. Modal predicate logic is one widely used variant which includes formulas such

1.Previous
3.Next