def do_OP_LEFT(vm):
    """
    >>> s = [b'abcdef', b'']
    >>> do_OP_LEFT(s, require_minimal=True)
    >>> print(len(s)==1 and s[0]==b'abc')
    True
    >>> s = [b'abcdef', b'']
    >>> do_OP_LEFT(s, require_minimal=True)
    >>> print(len(s) ==1 and s[0]==b'')
    True
    """
    pos = vm.pop_nonnegative()
    vm.append(vm.pop()[:pos])
