The CLOSE command breaks the connection between a process and a device.
The format of the CLOSE command is:
The optional truth-valued expression immediately following the command is a command postconditional that controls whether or not GT.M executes the command.
The required expression specifies the device to CLOSE.
The optional keywords specify device parameters that control device behavior; some device parameters take arguments delimited by an equal sign (=). If there is only one keyword, the surrounding parentheses are optional.
An indirection operator and an expression atom evaluating to a list of one or more CLOSE arguments form a legal argument for a CLOSE.
CLOSE of the current device ($IO), implicitly uses $PRINCIPAL.