  Computations with floats are only approximate, although they
are described as if the results were mathematically accurate.
Two mathematically identical expressions may be computationally
different because of errors inherent in the floating-point
approximation process. The precision of a float is not
necessarily correlated with the accuracy of that number. For
instance, 3.142857142857142857 is a more precise approximation
to <PI> than 3.14159, but the latter is more accurate. The
precision refers to the number of bits retained in the
representation. When an operation combines a short float with a
long float, the result will be a long float. Common Lisp
functions assume that the accuracy of arguments to them does not
exceed their precision. Therefore when two small floats are
combined, the result is a small float. Common Lisp functions
never convert automatically from a larger size to a smaller one.
Copyright 1996, The Harlequin Group Limited. All Rights
Reserved.