Some groups of operations can be generalized as following the same pattern. For
example, an operation setting Content-Type header to text/html is very
similar to setting the same header to application/json. We name plugs to this
level of abstraction on top of operations: plugs are operation builders. In
other words, they are higher-order functions that return functions.
Being just functions, we take as a convention that plugs respond to #call to
create an operation.
This library ships with some useful plugs.