The Walk action (line 1 below) presents a combination of
sequential and parallel movements. It is a sequence (line 2) of a
rotation towards the point to reach (line 3) followed by the actual
walking animation. The walking animation is built by parallelizing
(line 4) a looping walking cycle (line 5), with the primitive animation
moving the agent between two specified points (line 6). The
‘‘first’’ directive (line 8) says that the parallel animation terminates
as soon as the first of its component ends