reorganize positional methods in array.go, tests.

array.go

@@ -0,0 +1,31 @@
+package goquery
+
+// First() reduces the set of matched elements to the first in the set.
+// It returns a new Selection object.
+func (this *Selection) First() *Selection {
+	return this.Eq(0)
+}
+
+// Last() reduces the set of matched elements to the last in the set.
+// It returns a new Selection object.
+func (this *Selection) Last() *Selection {
+	return this.Eq(-1)
+}
+
+// Eq() reduces the set of matched elements to the one at the specified index.
+// If a negative index is given, it counts backwards starting at the end of the set.
+// It returns a new Selection object, and an empty Selection object if the index is invalid.
+func (this *Selection) Eq(index int) *Selection {
+	var l = len(this.Nodes)
+
+	if index < 0 {
+		index += l
+	}
+	return this.Slice(index, index+1)
+}
+
+// Slice() reduces the set of matched elements to a subset specified by a range of indices.
+// At the moment, negative indices are not supported.
+func (this *Selection) Slice(start int, end int) *Selection {
+	return pushStack(this, this.Nodes[start:end])
+}