The rules handling multiple interacting attack and block modifications
   have evolved throughout the game's history.  Rules 508 and 509 define
   legal attacks and blocks, here is a quick summary of the current state
   of affairs:

   When you declare an attack, you may pay any costs that are needed to
   allow creatures to attack, then pick a set of untapped creatures as
   the attacking creatures.  Check for any restrictions and requirements
   that are violated - if there are, you have to choose a different
   set of attackers.  To perform this check, first make sure that all
   of the restrictions are being met, then look at the requirements.
   If the maximum possible number of requirements are being followed,
   then the set of attackers is legal.

   Example:

   Player A controls:

   Crazed Goblin  {R}  Creature - Goblin Warrior  1/1
/ Crazed Goblin attacks each turn if able.

   Runeclaw Bear  {1}{G}  Creature - Bear  2/2

   Player B controls:

   Silent Arbiter  {4}  Artifact Creature - Construct  1/5
/ No more than one creature can attack each combat.
/ No more than one creature can block each combat.

   This situation has one requirement, the Crazed Goblin must attack.
   It also has one restriction, at most one creature can attack.

   The possible sets of attackers that Player A may declare are:

   - Just the Crazed Goblin: legal, as it satisfies the requirement
     without violating the restriction.

   - The Crazed Goblin and the Runeclaw Bear: illegal, as it does not
     satisfy the restriction.

   - Just the Runeclaw Bear: illegal, as attacking with just the Crazed
     Goblin is legal and satisfies more requirements.

   - No creatures: illegal, as attacking with just the Crazed Goblin is
     legal and satisfies more requirements.

   The situation is analogous with blockers: all of the restrictions
   must be met, and then a maximum number of requirements must also be
   met.  An example:

   Player A controls:

   Razorgrass Screen  {1}  Artifact Creature - Wall  2/1
/ Defender
/ Razorgrass Screen blocks each turn if able.

   Runeclaw Bear  {1}{G}  Creature - Bear  2/2

   Player B controls:

   Goblin War Drums  {2}{R}  Enchantment
/ Each creature you control can't be blocked except by two or more
   creatures.

   Player B is attacking with one creature.  Player A's blocking options
   are:

   - Both Razorgrass Screen and Runeclaw Bear: legal, as the requirement
     for the Razorgrass Screen to block is met, and the restriction for
     at least two creatures to block the attacker is also met.

   - Just the Razorgrass Screen: illegal; while the requirement is met,
     the restriction is not, and another legal block is available that
     meets the requirements and also satisfies the restrictions.

   - Just the Runeclaw Bear: illegal, as the requirement is not met, and
     another legal block is available that does meet the requirement.

   - Neither creature: illegal, as there is a restriction that could be
     met and the requirements can be met as well.