code: def do_OP_LEFT(vm):
      
      pos = vm.pop_nonnegative()
      vm.append(vm.pop()[:pos])
docs: >>> s = [b'abcdef', b'\3']
      >>> 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