The Frequency trait represents types that keep track of the
observed counts of items.
Add frequency to your Cargo.toml:
[dependencies]
frequency = "^1.0.0"
To use the Frequency interface for types implementing Frequency,
you must import the Frequency trait:
``` extern crate frequency;
use frequency::Frequency; ```
Implementations of Frequency are provided by the
frequency-btreemap,
frequency-btreemap, and
frequency-btreemap crates.