order_by_key Interface

# `order_by_key` Interface

Finds an order corresponding to an entry within the sequence.

1. `order_statistics_key.hpp`
2. Template parameters.
3. Public Types and Constants:
   1. Key-type definitions.
   2. Container definitions.
4. Public Methods:
   1. Operators.

## Template parameters.

Parameter | Description | Default Value
    Cntnr
| Container type. | -

## Key-type definitions.

Type | Definition | Description
    order_statistics_key_type
    typename Cntnr::key_type
| Order-statistics key type.
    underlying_key_type
    typename order_statistics_key_type::key_type
| Underlying key type.

## Container definitions.

Type | Definition | Description
    cntnr
    Cntnr
| Container type.
    size_type
    typename cntnr::size_type
| Container's size type.

## Operators.

Method | Description
    inline size_type
      operator()
      (const Cntnr &r_c,
        const underlying_key_type &r_key) const
| Returns the order of a key within a sequence. For exapmle, if `r_key` is the smallest key in `r_c`, this method will return 0; if `r_key` is a key between the smallest and next key in `r_c`, this method will return 1; if `r_key` is a key larger than the largest key in `r_c`, this method will return the size of `r_c`.