If the tracing flag is clear, tron does nothing. Otherwise, tron produces a diagnostic of the form “[line]” where line is the line number of the statement currently being executed. The diagnostic is not terminated with a newline. If no line number was given for the BASIC statement, line appears as 0 (zero). Calls to this function are inserted at the beginning of the code for every executable BASIC statement if any TRON or TROFF statements appear in the BASIC module.
These functions implement the TRON and TROFF statements. The BASIC compiler forbids the use of these statements if basic(1) was invoked with the -L switch.
from The Basmark QuickBASIC Programmer’s Manual by Lawrence Leinweber