How to copy operator definitions from amsopn.sty?

I want to create an operator which looks exactly like one defined in the amsmath package. I’ve found the relevant definition in amsopn.sty, but when I copy it into the preamble of my document I get an error message Use of varcolim doesn't match its definition. The code is below.

I also welcome suggestions on how better to do this. I’d rather use a "higher level" command to create my desired symbol, but I don’t like what newcommandvarinjcolim{operatornamewithlimits{underrightarrow{mathrm{colim}}}} produces (its spacing isn’t as tight as varinjlim, and I’d really like it to be identical to that except from saying "colim" instead of "lim").

documentclass{article}

usepackage{amsmath}

defvarcolim@#1#2{%
  vtop{m@thialign{##cr
    hfil$#1operator@font colim$hfilcr
    noalign{nointerlineskipkern1.5ex@}#2cr
    noalign{nointerlineskipkern-ex@}cr}}%
}
defvarinjcolim{%
  mathop{mathpalettevarcolim@{rightarrowfill@textstyle}}nmlimits@
}

begin{document}

$varinjlim$

$varinjcolim$

end{document}