"don't interact" is a strong statement. The compiler simply checks that each access to 'oscar' in the call to 'balance' can only modify independent pieces of memory ('heath' vs 'energy'). This is a special case because any access to a struct is generally modeled as a read/write to the entire struct value. The compiler does a little extra work to detect and allow this special case because copying the 'oscar' struct in situations like this would be inconvenient.
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