;;; Calculate the curl of a 3-d vector field
function curl_r,f,rr_1,cot_theta
COMPILE_OPT IDL2,HIDDEN
 return,yder(f[*,*,*,2])-zder(f[*,*,*,1])+cot_theta*rr_1*f[*,*,*,2]
end
function curl_theta,f,rr_1
COMPILE_OPT IDL2,HIDDEN
 return,zder(f[*,*,*,0])-xder(f[*,*,*,2])-rr_1*f[*,*,*,2]
end
function curl_phi,f,rr_1
COMPILE_OPT IDL2,HIDDEN
 return,xder(f[*,*,*,1])-yder(f[*,*,*,0])+rr_1*f[*,*,*,1]
end
function curl_sph,f,rr_1,cot_theta
COMPILE_OPT IDL2,HIDDEN
 w=make_array(size=size(f),/nozero)
 w[*,*,*,0]=curl_r(f,rr_1,cot_theta)
 w[*,*,*,1]=curl_theta(f,rr_1)
 w[*,*,*,2]=curl_phi(f,rr_1)
 return,w
end