Description:
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Currently it's impossible to clear the first bit only. Calling BitSet::clear(0) clears the total bitset, which should not happen and is inconsistent with BitSet::set(0). I've prepared a pull request to address this issue at github.

Test script:
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<?php

$a = new BitSet();
$a->set(0, 1);
$a->clear(0);
var_dump((string)$a);


Expected result:
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string(64) "0100000000000000000000000000000000000000000000000000000000000000"

Actual result:
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string(64) "0000000000000000000000000000000000000000000000000000000000000000"