Returns the index of the first entry in the list that matchs value a, starting at index istart.
If istart is not provided, the search starts at the first index 1.
If no match is found, the return value is 0.
Returns:
i - The first index matching a, or 0 if none is found.
Arguments:
l - The list to search.
a - The entry to find in the list.
istart - The index to start the search at.
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