How to tell if highlight has been defined

Consider the highlight

:hi MyHi ctermbg=46 guibg=#00ff00

In a script, is it possible to test whether or not its
been defined.

If one uses:

:hi MyHi

it will produce an Error message or the highlight terms.
What is want is something like:

if ! defined_highlight("MyHi")
:hi MyHi ....
endif

Thanks
Richard
--
Quis custodiet ipsos custodes

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