The Let Command
===============

If you have an expression like `a+b^2' on the stack and you wish to
compute its value where `b=3', you can simply store 3 in `b' and then
press `=' to reevaluate the formula.  This has the side-effect of
leaving the stored value of 3 in `b' for future operations.

The `s l' (`calc-let') command evaluates a formula under a
*temporary* assignment of a variable.  It stores the value on the
top of the stack into the specified variable, then evaluates the
second-to-top stack entry, then restores the original value (or lack of one)
in the variable.  Thus after `' a+b^2 RET 3 s l b RET',
the stack will contain the formula `a + 9'.  The subsequent command
`5 s l a RET' will replace this formula with the number 14.
The variables `a' and `b' are not permanently affected in any way
by these commands.

The value on the top of the stack may be an equation or assignment, or
a vector of equations or assignments, in which case the default will
be analogous to the case of `s t RET'.  See Storing Variables.

Also, you can answer the variable-name prompt with an equation or
assignment: `s l b=3 RET' is the same as storing 3 on the stack and
typing `s l b RET'.

The `a b' (`calc-substitute') command is another way to substitute a
variable with a value in a formula.  It does an actual substitution
rather than temporarily assigning the variable and evaluating.  For
example, letting `n=2' in `f(n pi)' with `a b' will produce `f(2 pi)',
whereas `s l' would give `f(6.28)' since the evaluation step will also
evaluate `pi'.