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.