INTERVAL _ _ _ _ _ _ _ _ _ _ _ _ type
Intervals are generally coded as lower bound and upper bound connected by the operator .., usually associated to a variable in an equation.
where <var> is a kernel and <low>, <high> are numbers or expression which evaluate to numbers with <low><=<high >.
means that the variable x is taken in the range from 2 .5 up to 3.5.