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diff --git a/autoload/fieldmarshal.vim b/autoload/fieldmarshal.vim
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+
+" Modify the text described by the '[ and '] marks.
+"
+" @param cb the callback used to modify the text should have the following
+"        function:
+"
+"          {operate} - takes a list and returns the modified list.
+"
+"
+" This function does make temporary use of the @a register, and thus the operate
+" function should not expect changes to that register to persist.
+function! fieldmarshal#modifytext(type, cb) abort
+  let save_a = getreg('a')
+  let save_a_type = getregtype('a')
+
+  " Yank the contents of the described text object into the "a register.
+  if a:type == 'line'
+    let vis = "'[V']"
+  elseif a:type == 'block'
+    let vis = "`[\<C-v>`]"
+  else
+    let vis = "`[v`]"
+  endif
+  silent! exec printf("norm! %s\"ay", vis)
+
+  " Get the a register, 
+  let ls = getreg('a', 1, !get(a:cb, 'as_chars', 0))
+  let regtype = getregtype('a')
+
+  let ls = a:cb.operate(ls, regtype)
+
+  " Set the register list to whatever was returned from operate(). Then
+  " re-VIsualize the text object and paste.
+  call setreg('a', ls, regtype)
+  silent! exec "norm! gv\"ap"
+
+  " Restore the register "a to what it was before.
+  call setreg('a', save_a, save_a_type)
+endfunction