A $\_\_\_\_\_\_\_\_\_\_$

is a Boolean variable or its complement. A

of the Boolean variables x$1,$ x$2,...,$ xn is a Boolean product y$1$y$2$yn where yi $=$ xi or yi $=$ x

i$.$ Such a product $($that is$,$ a product of n terms where for each of the n Boolean variables the variable itself or its complement is included$)$ has a value $1$ for exactly

combination of values of its variables.