Support modulus operation (absolute value)

This adds support for computing the absolute value of a term along with
an according unit test.

tests/TestCompletion.cpp

@@ -184,4 +184,12 @@ TEST_CASE("[completion] Rules are completed", "[completion]")
 
 		CHECK(output.str() == "forall V1, V2 not color(V1, V2)\nforall U1, U2, U3 (not color(U1, U2) or not color(U1, U3) or U2 = U3)\n");
 	}
+
+	SECTION("absolute value operation")
+	{
+		input << "adj(X, Y) :- X = 1..n, Y = 1..n, |X - Y| = 1.";
+		anthem::translate("input", input, context);
+
+		CHECK(output.str() == "forall V1, V2 (adj(V1, V2) <-> (V1 in 1..n and V2 in 1..n and |(V1 - V2)| = 1))\n");
+	}
 }