(adapted from useful-subroutines.ngc)
(Original file is Copyright 2006 Chris Radek and released under the GPL v2.)

(info: slot: specify x1,y1 and x2,y2, applies radius compensation)
o<slot> sub

#<toolno>    =  #1 (=1)
#<rpm>       =  #2 (=2000)
#<feedrate>  =  #3 (=10)
#<zincr>     =  #4
#<cutdepth>  =  #5
#<zsafe>     =  #6 (=0.5)
#<zstart>    =  #7 (=0.005)
#<x1>        =  #8 (x1 center)
#<y1>        =  #9 (y1 center)
#<x2>        = #10 (x2 center)
#<y2>        = #11 (y2 center)
#<width>     = #12
#<scale>     = #13 (=1)
#<rotate>    = #14 (=0)
#<xoff>      = #15 (=0)
#<yoff>      = #16 (=0)
#<spin_notify> = #17 (=0)
#<use_g43>     = #18 (=1)
#<h_for_g43>   = #19 (=0)
#<verbose>     = #20 (=0)

o<i0> if [#<scale> EQ 0]
        #<scale> = 1.0
        (debug, scale was 0, setting #<scale>)
o<i0> endif

#<width> = [#<width> * #<scale>]

o<loadtool> call [#<toolno>][#<use_g43>][#<h_for_g43>][#<verbose>]
#<tooldiam>   = [#5410 + .001]

o<move> call [#<x1>][#<y1>][#<rotate>][#<scale>][#<xoff>][#<yoff>]
#<x1>      = #<_move:x>
#<y1>      = #<_move:y>
#<len1>    = #<_move:len>
#<phi1>    = #<_move:phi>
o<move> call [#<x2>][#<y2>][#<rotate>][#<scale>][#<xoff>][#<yoff>]
#<x2>      = #<_move:x>
#<y2>      = #<_move:y>
#<len2>    = #<_move:len>
#<phi2>    = #<_move:phi>


#<radius>=[#<width>/2] (radius)
#<backdir>=[[atan [#<y2>-#<y1>]/[#<x2>-#<x1>]] + 180] ("backward" direction)
#<rtdir>=[#<backdir> + 90] ("right" side direction)
#<rcos_bd>=[#<radius> * [cos [#<backdir>]]]
#<rsin_bd>=[#<radius> * [sin [#<backdir>]]]
#<rcos_rd>=[#<radius> * [cos [#<rtdir>]]]
#<rsin_rd>=[#<radius> * [sin [#<rtdir>]]]


        s #<rpm> m3
o<if1>   if [#<spin_notify> GT 0]
o<spin>    call [#<rpm>]
o<if1>   endif
        f#<feedrate>

        g0 z#<zsafe>
        g0 x[#<x1>+#<rcos_bd>+#<rcos_rd>] y[#<y1>+#<rsin_bd>+#<rsin_rd>]

        g41 g1 x[#<x1>+#<rcos_rd>] y[#<y1>+#<rsin_rd>]
        g1 z#<zstart>

        (#<dpercut>=[#<tooldiam>/2] rule-of-thumb)
        #<dpercut>=#<zincr>

        #<znow>=[#<zstart>-#<dpercut>] (z)
        (down toward the specified depth a bit at a time)

o201    while [#<znow> GT [0-#<cutdepth>]]
         g1 x[#<x2>+#<rcos_rd>] y[#<y2>+#<rsin_rd>] z#<znow>
         g3 x[#<x2>-#<rcos_rd>] y[#<y2>-#<rsin_rd>] r#<radius>
         g1 x[#<x1>-#<rcos_rd>] y[#<y1>-#<rsin_rd>]
         g3 x[#<x1>+#<rcos_rd>] y[#<y1>+#<rsin_rd>] r#<radius>
         #<znow>=[#<znow>-#<dpercut>]
o201    endwhile
         (down to the actual depth and one more cut)
         g1 x[#<x2>+#<rcos_rd>] y[#<y2>+#<rsin_rd>] z[0-#<cutdepth>]
         g3 x[#<x2>-#<rcos_rd>] y[#<y2>-#<rsin_rd>] r#<radius>
         g1 x[#<x1>-#<rcos_rd>] y[#<y1>-#<rsin_rd>]
         g3 x[#<x1>+#<rcos_rd>] y[#<y1>+#<rsin_rd>] r#<radius>
         g1 x[#<x2>+#<rcos_rd>] y[#<y2>+#<rsin_rd>]

         g0 z#<zsafe>
         g40
o<slot> endsub