{{alias}}( [mask,] [options] )
    Returns the current process mask, if not provided a mask; otherwise, sets
    the process mask and returns the previous mask.

    A mask is a set of bits, each of which restricts how its corresponding
    permission is set for newly created files.

    On POSIX platforms, each file has a set of attributes that control who can
    read, write, or execute that file. Upon creating a file, file permissions
    must be set to an initial setting. The process mask restricts those
    permission settings.

    If the mask contains a bit set to "1", the corresponding initial file
    permission is disabled. If the mask contains a bit set to "0", the
    corresponding permission is left to be determined by the requesting process
    and the system.

    The process mask is thus a filter that removes permissions as a file is
    created; i.e., each bit set to a "1" removes its corresponding permission.

    In octal representation, a mask is a four digit number, e.g., 0077,
    comprised as follows:

    - 0: special permissions (setuid, setgid, sticky bit)
    - 0: (u)ser/owner permissions
    - 7: (g)roup permissions
    - 7: (o)thers/non-group permissions

    Octal codes correspond to the following permissions:

    - 0: read, write, execute
    - 1: read, write
    - 2: read, execute
    - 3: read
    - 4: write, execute
    - 5: write
    - 6: execute
    - 7: no permissions

    If provided fewer than four digits, the mask is left-padded with zeros.

    Note, however, that only the last three digits (i.e., the file permissions
    digits) of the mask are actually used when the mask is applied.

    Permissions can be represented using the following symbolic form:

        u=rwx,g=rwx,o=rwx

    where

    - u: user permissions
    - g: group permissions
    - o: other/non-group permissions
    - r: read
    - w: write
    - x: execute

    When setting permissions using symbolic notation, the function accepts a
    mask expression of the form:

        [<classes>]<operator><symbols>

    where "classes" may be a combination of

    - u: user
    - g: group
    - o: other/non-group
    - a: all

    "symbols" may be a combination of

    - r: read
    - w: write
    - x: execute
    - X: special execute
    - s: setuid/gid on execution
    - t: sticky

    and "operator" may be one of

    - `+`: enable
    - `-`: disable
    - `=`: enable specified and disable unspecified permissions

    For example,

    - `u-w`: disable user write permissions
    - `u+w`: enable user write permissions
    - `u=w`: enable user write permissions and disable user read and execute

    To specify multiple changes, provide a comma-separated list of mask
    expressions. For example,

        u+rwx,g-x,o=r

    would enable user read, write, and execute permissions, disable group
    execute permissions, enable other read permissions, and disable other
    write and execute permissions.

    The `a` class indicates "all", which is the same as specifying "ugo". This
    is the default class if a class is omitted when specifying permissions. For
    example, `+x` is equivalent to `a+x` which is equivalent to `ugo+x` which
    is equivalent to `u+x,g+x,o+x` and enables execution for all classes.

    Parameters
    ----------
    mask: integer|string (optional)
        Mask or mask expression. If the mask is a string, the mask is assumed to
        be in symbolic notation.

    options: Object (optional)
        Options.

    options.symbolic: boolean (optional)
        Boolean indicating whether to return the mask using symbolic notation.

    Returns
    -------
    mask: integer|string
        Process mask. If provided a mask, the returned value is the previous
        mask; otherwise, the returned value is the current process mask.

    Examples
    --------
    > var mask = {{alias}}()
    <number>
    > mask = {{alias}}( { 'symbolic': true } )
    <string>

    See Also
    --------