## zoom

Only show selected terms in a sum, and restrict subsequent algorithms to these terms.

Often you want manipulations to only apply to a selected subset of terms in a large sum. The `zoom`

algorithm makes only certain terms visible, representing the remaining terms with dots.
Here is an expression with a 5 terms,ex:=\int{ A_{m n} + B_{m n} C + D_{m} F_{n} C + T_{m n} + B_{m n} R}{x};

\(\displaystyle{}\int \left(A_{m n}+B_{m n} C+D_{m} F_{n} C+T_{m n}+B_{m n} R\right)\,\,{\rm d}x\)

\int{A_{m n} + B_{m n} C + D_{m} F_{n} C + T_{m n} + B_{m n} R}{x}

In order to restrict attention only to the terms containing a $B_{m n}$ factor, we use

zoom(_, $B_{m n} Q??$);

\(\displaystyle{}\int \left( ... +B_{m n} C+ ... +B_{m n} R\right)\,\,{\rm d}x\)

\int{ ... + B_{m n} C + ... + B_{m n} R}{x}

Subsequent algorithms only work on the visible terms above, not on the terms hidden inside the dots,

substitute(_, $C->Q$);

\(\displaystyle{}\int \left( ... +B_{m n} Q+ ... +B_{m n} R\right)\,\,{\rm d}x\)

\int{ ... + B_{m n} Q + ... + B_{m n} R}{x}

To make the hidden terms visible again, use

`unzoom`

, and note that the third term below has remained unaffected
by the substitution above,unzoom(_);

\(\displaystyle{}\int \left(A_{m n}+B_{m n} Q+D_{m} F_{n} C+T_{m n}+B_{m n} R\right)\,\,{\rm d}x\)

\int{A_{m n} + B_{m n} Q + D_{m} F_{n} C + T_{m n} + B_{m n} R}{x}

The

`zoom`

/`unzoom`

combination is somewhat similar to the old deprecated `take_match`

/`replace_match`

algorithms, but makes it more clear that terms have been suppressed.