Checks for particular control characters may be suppressed by assigning the NULL character to them. The NULL character is itself a control character which may be assigned a value using this routine. Some control characters form pairs, and an error is reported if only one member of a pair is assigned the NULL value. These pairs are OPEN_NEST and CLOSE_NEST, and OPEN_KERNEL and CLOSE_KERNEL.

If a blank value for argument CCLIST is supplied, then the default control characters described in the "Notes" section are re-established.

An error is reported if any two control characters are the same. The exception to this is that any number of control characters may have the same value as the NULL control character. If any error occurs, the control characters are left unaltered.

IGRP = INTEGER (Given)

CCLIST = CHARACTER $\ast$ ( $\ast$ ) (Given)

CC = CHARACTER $\ast$ ( $\ast$ ) (Given)

STATUS = INTEGER (Given and Returned)

• INDIRECTION: (Default "$$^") The name given to the character used to indicate that an element of a group expression is the name of a text file from which further elements should be read.

• COMMENT: (Default "#") The name given to the character used to introduce comments in group expressions.

• DELIMITER: (Default ",") The name given to the character used to delimit elements within group expressions. Note, delimiters within group expressions are ignored if they occur within matched nesting characters (see OPEN_NEST and CLOSE_NEST below).

• NAME_TOKEN: (Default "$\ast$") The name given to the character used as a token for input names in a modification element.

• SEPARATOR: (Default "$|$") The name given to the character used to separate the substitution strings within a modification element.

• OPEN_NEST: (Default "(") The name given to the character used to open a "nest" within a group expression. Any delimiter characters occurring within matched nesting characters are ignored.

• CLOSE_NEST: (Default ")") The name given to the character used to close a "nest" within a group expression.

• FLAG: (Default "-") The name given to a character which can be appended to the end of a group expression in order to "flag" that expression. The interpretation of this flag is left up to the application.

• OPEN_KERNEL: (Default "{") The name given to the character used to open a "kernel" within a group expression.

• CLOSE_KERNEL: (Default "}") The name given to the character used to close a "kernel" within a group expression.

• NULL: (Default "%") The name given to the character which can be assigned to other control characters to suppress checks for those control characters. If this is changed, any other characters currently set to the null character are also changed to the new NULL character.

• ESCAPE: (Default to the NULL character) The name given to the character which can be used to escape control characters within a group expression.