The USE command selects the current device for READs (input) and WRITEs (output).
The format of the USE command is:
U[SE][:tvexpr] expr[:(keyword[=expr][:...])][,...]
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 make the current device.
A USE that selects a device not currently OPENed by the process causes a run-time error.
The optional keywords specify deviceparameters that control device behavior; some deviceparameters take arguments delimited by an equal sign (=); if the argument only contains one deviceparameter, the surrounding parentheses are optional.
An indirection operator and an expression atom evaluating to a list of one or more USE arguments form a legal argument for a USE.
