If he slapped the wall beyond the foul line, call him on it. It is a rule for a reason, and we can either abide by them, or we can put up with the chaos from people choosing only to abide by SOME of the rules. And... ignorance of the rules is not an excuse. I am always amazed when veteran bowlers (10+ years) come to me with a rules question, only to discover that most have never attempted to read the rule book. The rule book is not that long, and the language is easy to read. C'mon people.... at least make an ATTEMPT at knowing the rules. You might be surprised by some of the things you discover!
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r.k.wolfe