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# Lambda Expression

### Description

A Lambda Expression is a compile-time invocable entity in either of the following two forms:

- Metafunction Class
- Placeholder Expression

Most of the MPL components accept either of those, and the concept gives us a consice way to describe these requirements.

### Expression requirements

See corresponding Metafunction Class and Placeholder Expression specifications.

### Models

- `always`
- `unpack_args`
- `plus<_, int_<2> >`
- `if_< less<_1, int_<7> >, plus<_1,_2>, _1 >`

### See also

Metafunctions, Placeholders, `apply`, `lambda`