Represents a number of special constant values defined by some G-code controllers:

• iterations - the number of completed iterations of the innermost loop.
• line - the current line number in the file being executed.
• null - the null object.
• pi - the constant π.
• result - 0 if the last G-, M- or T-command on this input channel was successful, 1 if it returned a warning, 2 if it returned an error.