ABSTRACT
To manipulate multidimensional ubounds (that is, uboxes), we need support functions that promote and demote the fraction size and exponent size. These must be used with care since they affect the ULP size, but they are useful when we can determine which end of the ULP-wide interval moves and which end stays put. Since the first two routines require exact unums as input, nothing will happen to the value they represent; they simply decompress the fraction field or exponent field by one bit. The reason it is more complicated to promote the exponent field is because subnormals behave differently from normals, and there is also the possibility that exponent promotion causes a subnormal to be expressible as a normal.
