## `id` -- the identity map

#### Introduction

`id(`x`)` evaluates and returns `x`.

#### Call(s)

`id(x)`
`id(x1, x2...)`

#### Parameters

`x, x1, x2, ...` | - | arbitrary MuPAD objects

#### Returns

the sequence of the input parameters.

#### Details

- `id(`x`)` evaluates and returns `x`; `id(`x1, x2, ...`)` returns the evaluated arguments as an expression sequence; `id(``)` returns the void object `null()`.
- `id` is a function of the system kernel.

#### Example 1

`id` returns the evaluated arguments:

    >> a := 2: id(a + 2)

                                         4

    >> id(a, b, 4 + 2)

                                      2, b, 6

`id(``)` returns `null()`:

    >> domtype(id())

                                     DOM_NULL

    >> delete a:

#### Example 2

`id` is useful when working with functional expressions:

    >> f := 3*id + sin + 5*id^2 + exp@(-id^2): f(x)

                                            2          2
                          3 x + sin(x) + 5 x  + exp(- x )

    >> D(f)

                                                    2
                         10 id + cos - 2 id exp@- id  + 3

    >> delete f:

#### Changes

- no changes

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