:   '''[dict] incr''' ''variable'' ''key'' ?''increment''?

This adds the given ''increment'' value (an integer that
defaults to 1 if not specified) to the value that the given
''key'' maps to in the [dictionary] value contained in the given
''variable'', writing the resulting dictionary value back to
that variable. Non-existent keys are treated as if they map
to 0. It is an error to increment a value for an existing
key if that value is not an integer.

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**See also**
   * [incr]

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