z is not equal to 0, is z^2/[z] < 1? =absolute value
1) z <1
2) z >-1
In order to yield a value that is less than 1, the denominator has to be greater than the numerator.
This is the case if -1<z<1, because the square of a number between -1 and 1 is smaller than the original number.
Both statement are needed in order to assure that z is between -1 and 1.