Add missing snippets for erf/erfc/lgamma functions.
diff --git a/doc/snippets/Cwise_erf.cpp b/doc/snippets/Cwise_erf.cpp
new file mode 100644
index 0000000..7f51c1b
--- /dev/null
+++ b/doc/snippets/Cwise_erf.cpp
@@ -0,0 +1,2 @@
+Array4d v(-0.5,2,0,-7);
+cout << v.erf() << endl;
diff --git a/doc/snippets/Cwise_erfc.cpp b/doc/snippets/Cwise_erfc.cpp
new file mode 100644
index 0000000..f0453d4
--- /dev/null
+++ b/doc/snippets/Cwise_erfc.cpp
@@ -0,0 +1,2 @@
+Array4d v(-0.5,2,0,-7);
+cout << v.erfc() << endl;
diff --git a/doc/snippets/Cwise_lgamma.cpp b/doc/snippets/Cwise_lgamma.cpp
new file mode 100644
index 0000000..cbc69b9
--- /dev/null
+++ b/doc/snippets/Cwise_lgamma.cpp
@@ -0,0 +1,2 @@
+Array4d v(0.5,10,0,-1);
+cout << v.lgamma() << endl;
\ No newline at end of file