Controlling the display of a syntax highlighting group

To find non-ascii characters in a file, I have defined the following:

    syntax match nonascii "[^\x00-\x7F]"

    highlight nonascii ctermbg=13

I can disable this with the command:

    syntax off

I would like to create a command that toggles this highlight group on/off and

hopefully do so without affecting the current state of the remainder of the

syntax highlight groups.  I have not found a way to test the state of this

group, and once activated, I cannot seem to clear or edit the highlight

definition non-destructively.

Any pointers?

Thanks,
--
Jeff

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