add more help text

cmd/stirling.go

@@ -58,10 +58,24 @@ func stirling(cmd *cobra.Command, args []string) {
 
 // stirlingCmd represents the stirling command
 var stirlingCmd = &cobra.Command{
-	Use:   "stirling",
+	Use:   "stirling [-1|-2] -n N -k N",
 	Short: "Compute the Stirling numbers",
-	Long:  `Compute the Stirling numbers.`,
-	Run:   stirling,
+	Long: `Compute the Stirling numbers.
+
+The Stirling numbers of the first kind give the coefficients in the expansion of the falling factorial. For example,
+x(x-1)(x-2) = x^3 - 3x^2 + 2x
+Consequently, s(3,1) = 2, s(3,2) = -3, and s(3,3) = 3.
+
+The Stirling numbers of the second kind count the number of ways to partition a set of n objects into k non-empty subsets. For example, there are 7 ways to partition a set of 4 objects into 2 non-empty subsets:
+{1} {2,3,4}
+{2} {1,3,4}
+{3} {1,2,4}
+{4} {1,2,3}
+{1,2} {3,4}
+{1,3} {2,4}
+{1,4} {2,3}
+Therefore S(4,2) = 7.`,
+	Run: stirling,
 }
 
 func init() {