"With potential budget and performance challenges in implementing the Vision for Space Exploration, it will be necessary to make some changes to the traditional twelve days of Christmas. All organizations are asked to implement the following immediately:
1) The "partridge in a pear tree" is a classic example of conflicting requirements that cannot be satisfied concurrently: Either the partridge will eat the pears, rendering the tree ineffective, or it will not, and the bird will die. Ongoing continuous replacement of partridges and/or pears would be the inevitable outcome of deployment. The Constellation program is directed to deploy either a partridge or a pear tree, but not both. ..."