diff --git a/analyse/analyse.go b/analyse/analyse.go
index 0090725..384dd3b 100644
--- a/analyse/analyse.go
+++ b/analyse/analyse.go
@@ -35,6 +35,7 @@ func (ws wordWeights) Swap(i, j int) {
 	ws[i], ws[j] = ws[j], ws[i]
 }
 
+// Keyword extraction.
 func ExtractTags(sentence string, topK int) (tags wordWeights) {
 	freq := make(map[string]float64)
 
diff --git a/analyse/textrank.go b/analyse/textrank.go
index 889308d..21819de 100644
--- a/analyse/textrank.go
+++ b/analyse/textrank.go
@@ -113,6 +113,8 @@ func (u *undirectWeightedGraph) rank() wordWeights {
 	return result
 }
 
+// Extract keywords from sentence using TextRank algorithm. the allowed POS list
+// could be manually speificed.
 func TextRankWithPOS(sentence string, topK int, allowPOS []string) wordWeights {
 	posFilt := make(map[string]int)
 	for _, pos := range allowPOS {
@@ -152,6 +154,8 @@ func TextRankWithPOS(sentence string, topK int, allowPOS []string) wordWeights {
 	return tags
 }
 
+// Extract keywords from sentence using TextRank algorithm.
+// topK specify how many top keywords to be returned at most.
 func TextRank(sentence string, topK int) wordWeights {
 	return TextRankWithPOS(sentence, topK, defaultAllowPOS)
 }