turtles-on
a reporter that takes a set of agents (patches or turtles) as input and reports a set of all the turtles that are on (or standing on) the same patch(es)
pen-down, pen-erase, pen-up
a command that changes turtle’s mode to drawing lines (pen-down), removing lines (pen-erase), or neither (pen-up)
patch-ahead
a reporter that takes a distance (a number) as input and reports the patch that is that distance ahead of the current turtle (along the turtle’s current heading)
