Great tip, thanks a lot! You're right, I did not pay attention on this.

http://stackoverflow.com/questions/3230944/what-does-predicate-mean-in-the-context-of-computer-science
A predicate is a function that takes an input and returns a boolean as a filter criteria. Filtering is a common operation.
If you take a look at the test cases, the argument p supplied to your method is a predicate function, not an array.

Array.prototype.method = function ( a ) {
  return a
}

[ 1, 2, 3 ].method( 'arg' )   // returns 'arg'

The array it is called on is this

Array.prototype.method = function ( a ) {
  return this
}

[ 1, 2, 3 ].method( 'arg' )   // returns [ 1, 2, 3 ]