Maple's long history and extensive math library have invariably claimed some names that you may want to use for something else. For example, the capital letter 'I' in Maple means the imaginary unit, but why not the identity matrix instead? Similarly, after defining variables, A, B, and C, why can't you continue the pattern and use D? In Maple 17 you can.
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$\mathbf{local}\mathrm{I}\u2254\left[\begin{array}{rr}1& 0\\ 0& 1\end{array}\right]semi;$

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$\mathbf{local}\mathrm{D}:=5colon;$

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$\mathrm{D}\+\mathrm{D}$

${}$
For more information, see: Local Names