code: function( bodyA, bodyB, stiffness, targetLength ){
  
              var cst;
  
              if (!bodyA || !bodyB){
  
                  return false;
              }
  
              cst = {
                  id: Physics.util.uniqueId('dis-constraint'),
                  type: 'dis',
                  bodyA: bodyA,
                  bodyB: bodyB,
                  stiffness: stiffness || 0.5,
                  targetLength: targetLength || bodyB.state.pos.dist( bodyA.state.pos )
              };
  
              cst.targetLengthSq = cst.targetLength * cst.targetLength;
  
              this._distanceConstraints.push( cst );
              return cst;
          }
docs: VerletConstraintsBehavior#distanceConstraint( bodyA, bodyB[, stiffness, targetLength] ) -> Object
  - bodyA (Body): First body
  - bodyB (Body): Second body
  - stiffness (Number): A number between 0 and 1 that represents the stiffness of the constraint. Defaults to: `0.5`
  - targetLength (Number): Target length. defaults to current distance between the bodies
  + (Object): The constraint data object
  
  Constrain two bodies to a target relative distance.
  
  Returns constraint data that can be used to remove the constraint later.
  
  - `.bodyA` and `.bodyB` are references to the bodies
  - `.type` is the type of constraint
  - `.id` is the string ID of the constraint
  - `.stiffness` is the stiffness
  - `.targetLength` is the target length