Depending on your APL interpreter, you may be able to paste the
following text into your function editor to create this function:


Z„ANGLE M
© Given the slopes of two lines, <M>, compute the angle between
© the lines in radians.
© The following example gives a 90 degree result:
©     RTOD ANGLE 0 1E100 
–(¯1=×/M)/'…0,Z„±.5'
Z„¯3±|(-/M)÷1+×/M


In APL2000 or APL+Win you can copy the following functions to
the clipboard and then use the command:

  ]ASCII2APL /P

Z{<-}ANGLE M
 @ Given the slopes of two lines, <M>, compute the angle between
 @ the lines in radians.
 @
 @ The following example gives a 90 degree result:
 @
 @     RTOD ANGLE 0 1E100

 {execute}({neg}1={times}/M)/'{->}0,Z{<-}{pitimes}.5'
 Z{<-}{neg}3{circle}¦(-/M){divide}1+{times}/M



R{<-}DTOR D
 @ Convert degrees to radians

 R{<-}{pitimes}D{divide}180



Z{<-}RTOD R
 @ Convert radians to degrees.

 Z{<-}R{times}180{divide}{pitimes}1