Functional sequences can be subset using single or double brackets.
A single-bracket subset results in a new functional sequence, and
a double-bracket subset results in a single function.

## Usage

```
# S3 method for fseq
[[(x, ...)
# S3 method for fseq
[(x, ...)
```

## Arguments

- x
A functional sequence

- ...
index/indices. For double brackets, the index must be of length 1.

## Value

A function or functional sequence.