diff --git a/src/type.hpp b/src/type.hpp
index f95c0ca..7a365a7 100644
--- a/src/type.hpp
+++ b/src/type.hpp
@@ -1,6 +1,9 @@
+#pragma once
 #include <complex>
 /// the datatype for all template instantiations
 using dtype = std::complex<double>;
 
+constexpr static dtype Imag(0, 1.0);
+
 // template<int N>
 // using Tensor = std::shared_ptr<e::Tensor<dtype, N>>;