The replacement operator /. allows you to apply transformation
rules to a specific expression. Often, however, you want to have
transformation rules automatically applied whenever possible.
You can do this by assigning explicit values to Mathematica
expressions and patterns. Each assignment specifies a
transformation rule to be applied whenever an expression of the
appropriate form occurs. |expr/.lhs->rhs||apply a transformation
rule to a specific expression| |lhs=rhs||assign a value which
defines a transformation rule to be used whenever possible|
Manual and automatic application of transformation rules. This
applies a transformation rule for x to a specific expression. By
assigning a value to x , you tell Mathematica to apply a
transformation rule for x is transformed automatically. You
should realize that except inside constructs like Module , all
assignments you make in a Mathematica session are permanent .
They continue to be used for the duration of the session, unless
you explicitly clear or overwrite