# `Expresso.Element.Math.Links`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/element/math/links.ex#L1)

The links between the lines of a derivation

`\from{name}{…}` in the LaTeX of a line marks a sub-term that comes from the
part `name` of an earlier line. Temml writes the attribute `data-from` on
the MathML of the sub-term. `place/1` gives each link a copy of its source,
with the copies of `Expresso.Element.Copy`. The copy has the steps of the
line of the link, and it moves from its source to the linked sub-term.

The source of a link is each element with the name in the nearest earlier
line that has the name. An element belongs to the line of its nearest
ancestor with the name of a line. `docs/line-links.md` gives the design.

A link with the name of a set of `Expresso.Element.Math.Each` takes one
occurrence of the set: the link number `k` with that name and that source
line takes the occurrence `k` of the set in that line.
`docs/each-occurrence.md` gives the design.

# `place`

```elixir
@spec place(Expresso.Element.Math.t()) :: Expresso.Element.Math.t()
```

Give each link of a math element a copy of its source

The function gives the linked sub-term number `n` the class `link-n`, and
each element of a source in the line `j` the class `from-j-name`. It then
adds one copy part for each link: from `from-j-name` to `link-n`, with the
steps of the part of the line of the link. It raises for a name that no
earlier line has, and for a line with links and with no steps.

A line with `steps: 2` shows from its second step, and its copies move from
its first step.

A line of a rewrite takes a step for each group of its phases. The
`rewrite` of the line gives the step when each part gets its color, such as
the occurrences of the rewrite in the old line and the source of `by`, the
step when the copies of the kept runs, `keep-k-n`, move, and the step when
the other copies move. The line shows at its last step.

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*Consult [api-reference.md](api-reference.md) for complete listing*