5.29 Traps

Traps are locations in the output, or conditions on the input that, when reached or fulfilled, call a specified macro. These traps can occur at a given location either on the page or in the current diversion (together, these are known as vertical position traps), at a blank line, at a line with leading space characters, after a quantity of input lines, or at the end of input. Setting a trap is also called planting one. It is said that a trap is sprung if its condition is fulfilled. The formatter passes no arguments to macros called by traps.