parameters: (a, b)
return_statement: return ordering
docstring: Find the left-most consistent ordering between two lists of unique items.
  
    A consistent ordering combines all elements in both a and b while keeping all
    elements in their original order in both inputs. The left-most consistent
    ordering orders elements from `a` not found in `b` before elements in `b` not
    found in `a`.
  
    For example, given ['x', 'z'] and ['y', 'z'], both ['x', 'y', 'z'] and ['y',
    'x', 'z'] are consistent orderings because each of the inputs appears in
    each consistent ordering in the same order, and ['x', 'y', 'z'] is the
    left-most, because 'x' appears only in `a` and 'y' appears only in `b`. In
    contrast, there is no consistent ordering between ['x', 'y'] and ['y', 'x'].
  
    Args:
      a: list with unique elements.
      b: list with unique elements.
  
    Returns:
      List containing all elements in either a or b, or None, if no consistent
      ordering exists.
docstring_summary: Find the left-most consistent ordering between two lists of unique items.
docstring_tokens: Find the left - most consistent ordering between two lists of unique items .
function: def _find_consistent_ordering(a, b):
    """Find the left-most consistent ordering between two lists of unique items.
  
    A consistent ordering combines all elements in both a and b while keeping all
    elements in their original order in both inputs. The left-most consistent
    ordering orders elements from `a` not found in `b` before elements in `b` not
    found in `a`.
  
    For example, given ['x', 'z'] and ['y', 'z'], both ['x', 'y', 'z'] and ['y',
    'x', 'z'] are consistent orderings because each of the inputs appears in
    each consistent ordering in the same order, and ['x', 'y', 'z'] is the
    left-most, because 'x' appears only in `a` and 'y' appears only in `b`. In
    contrast, there is no consistent ordering between ['x', 'y'] and ['y', 'x'].
  
    Args:
      a: list with unique elements.
      b: list with unique elements.
  
    Returns:
      List containing all elements in either a or b, or None, if no consistent
      ordering exists.
    """
    a_set = set(a)
    b_set = set(b)
    i = 0
    j = 0
    ordering = []
    while i < len(a) and j < len(b):
      if a[i] not in b_set:
        ordering.append(a[i])
        i += 1
      elif b[j] not in a_set:
        ordering.append(b[j])
        j += 1
      elif a[i] == b[j]:
        ordering.append(a[i])
        i += 1
        j += 1
      else:
        return None
  
    ordering.extend(a[i:])
    ordering.extend(b[j:])
  
    return ordering
function_tokens: def _find_consistent_ordering ( a , b ) : a_set = set ( a ) b_set = set ( b ) i = 0 j = 0 ordering = [ ] while i < len ( a ) and j < len ( b ) : if a [ i ] not in b_set : ordering . append ( a [ i ] ) i += 1 elif b [ j ] not in a_set : ordering . append ( b [ j ] ) j += 1 elif a [ i ] == b [ j ] : ordering . append ( a [ i ] ) i += 1 j += 1 else : return None ordering . extend ( a [ i : ] ) ordering . extend ( b [ j : ] ) return ordering