### atan

#### atan x y

Converts x and y offsets to a turtle heading in degrees (from 0 to 360).

Note that this version of atan is designed to conform to the geometry of the NetLogo world, where a heading of 0 is straight up, 90 is to the right, and so on clockwise around the circle. (Normally in geometry an angle of 0 is right, 90 is up, and so on, counterclockwise around the circle, and atan would be defined accordingly.)

When y is 0: if x is positive, it reports 90; if x is negative, it reports 270; if x is zero, you get an error.

    show atan 1 -1
    => 135
    show atan -1 1
    => 315
    crt 1 [ set heading 30  fd 1  print atan xcor ycor ]
    => 30

In the final example, note that the result of `atan` equals the turtle's heading.

If you ever need to convert a turtle heading (obtained with atan or otherwise) to a normal mathematical angle, the following should be helpful:

    to-report heading-to-angle [ h ]
      report (90 - h) mod 360
    end

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