### 9.5 Controlling Use of Common Denominators

When two rational functions are added, REDUCE normally produces an expression
over a common denominator. However, if the user does not want denominators
combined, he or she can turn off the switch MCD which controls this process. The latter
switch is particularly useful if no greatest common divisor calculations are desired, or
excessive differentiation of rational functions is required.

CAUTION: With MCD off, results are not guaranteed to come out in either normal or
canonical form. In other words, an expression equivalent to zero may in fact not be
simplified to zero. This option is therefore most useful for avoiding expression swell
during intermediate parts of a calculation.

MCD is normally on.