subroutine play_with_etd (nv,kaa,nkaa,nb,nu,gamma,freq,zamp)
      implicit complex (z)
      integer*2 kaa(0:nkaa,nv),kaat(0:9)
      integer   nb(nv),nu(nv)
      real      gamma(nv),freq(nv)
      complex   zamp(nv)
      character symb*4
      logical   enable,qq
      save

      data zi/(0.0,1.0)/, pi/3.1415926535/, enable/.false./
      
      if (.not.enable) return

      rad=pi/180.
c
c INVITATION: Add code between HERE >------------------------------>
c      
c This example changes the frequency of M4 to M6
c
c Note that SYMB must be a variable.
      symb='M4  '
c
c Find the location in the excitation spectrum.
c (KAAT does not have to be assigned values as the function is
c seeing a legitimate tide symbol in SYMB.) 
c NKAA = width of astro-argument-numbers list (7) and 
c NV   = length are handed over from the call list; don't change!  
      in=in_etd_spectrum(symb,kaat,kaa,nkaa,nv)
c
c Make a change at IN. IN will be -1 if the wave cannot be found. 
      if (in.gt.0) then
         freq(in)=1.5*freq(in)
         kaa(0,in)=6
         kaa(1,in)=6
         print *,'<Play-with-ETD>>> M4 changed to M6 in FREQ and KAA'
      endif
c
c <---------------------------------< and HERE <---------------------
      return
      
      entry enable_play_with_etd (qq)
      enable=qq
      return
      end