Idiom # 53: Join a list of strings
fold is the feature of choice to join elements of a set into a
value of the element type. For type string, such operations are defined in feature
strings. For concatenation with a separator,
sep is the operation to use.
Mathematically speaking, such an operation is called a monoid. This is
why the type of the argument of
the operations declared in
Since joining the elements of a list into a string using a given separator is a common operation, there is a convenience function to do this directly.