diff --git a/src/global.rs b/src/global.rs
index 3dddcd3..025b0f4 100644
--- a/src/global.rs
+++ b/src/global.rs
@@ -1,6 +1,7 @@
 
 pub const EULER: f64		=	2.718281828459045;
 pub const GOLDEN_RATIO: f64	=	1.618033988749895;
+pub const LIGHT_SPEED: f64	=	299792458f64;
 pub const PI: f64			=	3.141592653589793;
 pub const ROOT_TWO: f64		=	1.414213562373095;
 
diff --git a/src/helper.rs b/src/helper.rs
index 02c57de..221147a 100644
--- a/src/helper.rs
+++ b/src/helper.rs
@@ -15,7 +15,7 @@ pub fn fix(arg: &Value) -> EvalResult {
 
 	let count = args.len();
 	if count != 2 {
-		return Err(EvalexprError::WrongFunctionArgumentAmount { expected: 2, actual: count });
+		return Err(EvalexprError::WrongFunctionArgumentAmount { expected: 2..=2, actual: count });
 	}
 
 	let float = args[0].as_float()?;
@@ -63,7 +63,7 @@ pub fn logarithm(arg: &Value) -> EvalResult {
 			output = value.log(base).into();
 		},
 		_ => {
-			return Err(EvalexprError::WrongFunctionArgumentAmount { expected: 2, actual: count });
+			return Err(EvalexprError::WrongFunctionArgumentAmount { expected: 2..=2, actual: count });
 		}
 	}
 	return Ok(output);
diff --git a/src/main.rs b/src/main.rs
index c0a5395..ed5bc0a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -31,6 +31,7 @@ fn main() {
 	//	build eval context
 	let mut context = context_map! {
 		//	globals
+		"c"		=>	global::LIGHT_SPEED,
 		"e"		=>	global::EULER,
 		"phi"	=>	global::GOLDEN_RATIO,
 		"pi"	=>	global::PI,