`val (/:) : t -> ustring -> t`

`path/:name` is a path of `name` located in a directory `path`. For example:

- `PathType.root``/:"var"/:"log"` builds absolute path "/var/log"

- `[user]/:".ssh"` can be either:

  - absolute path "/.ssh" in case `user` is an empty string
  - relative path otherwise

`PathType.default_validator` is applied to the argument. `name` must not contain path separator (causes Illegal_char exception).

- raises `Illegal_char`

  (raised by validator on any bad character)

`val (//@) : t -> t -> t`

`basepath//\@relpath` catenates two paths.

Windows: As a special exception it is possible to pass absolute path as `relpath`, provided that `basepath` is simple absolute path (i.e. of the form `[...; ""]`) and `relpath` is not simple absolute path.

- raises `Invalid_argument`

  if the second argument is an absolute path (Windows: see above).