Customizing Maxima. Renaming Maxima functions

Dear List,

I would like to rename some Maxima commands somehow.  By rename I literally mean rename, the old name won't work anymore 
after it is done.  e.g.  I would like to change "integrate" to "rsum" for example.  So rsum(x^2,x) will return 1/3*x^3 
and rsum(x^2,x,0,1) will return 1/3 and also all error messages would be the same as if rsum were integrate.  Can 
someone help me?  Also undoing this would be good to know too so I can change the name in a block and then change it 
back at the end of the block for example.

Thanks,

Rich