STRING_IF -- conditional statement for strings

OPERATION = $\langle{\tt string:1}\rangle$	'SUM'	EQ | NE | INDEX
STRING_ARGUMENTS = $\langle{\tt string:0}\rangle$	''	arguments for string operation
THEN = $\langle{\tt string:1}\rangle$	'undefined'	statement when IF evaluates to T
ELSE = $\langle{\tt string:1}\rangle$	'undefined'	statement when IF evaluates to F

Description:

This command performs conditional IF operation on two string arguments. The possible operations are equal (EQ), not equal (NE), and the FORTRAN index() function (INDEX), which returns true if there is `argument2' substring within `argument1'. If the condition is true, the command specified in the THEN variable is executed. Otherwise the command in the ELSE variable is executed. Typically, these commands are GO_TO statements.