GNU Emacs Calc 2.02 Manual
11.1 Selecting Sub-Formulas
When working with an algebraic formula it is often necessary to
manipulate a portion of the formula rather than the formula as a
whole. Calc allows you to "select" a portion of any formula on
the stack. Commands which would normally operate on that stack
entry will now operate only on the sub-formula, leaving the
surrounding part of the stack entry alone.
One common non-algebraic use for selection involves vectors. To work
on one element of a vector in-place, simply select that element as a
"sub-formula" of the vector.