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Constant Propagation
--------------------
Constant propagation is a code reduction technique in which values of variables
which are determined to be constants can be passed to expressions which use
these constants and can be computed at compile time.


Example:

Consider the following segment of an original source program:

      x = 7;
      y = 2 * x;
      z = f (x, y);

The value of variable x in the segment's last two lines is the constant number 7.
This is always true, because of the assignment to x in the segment's first line.
Therefore, the two instances of x can be substituted by the constant value 7
(this value propagates through the next two lines). The reduced source program
is the following:

      x = 7;
      y = 2 * 7;
      z = f (7, y);