diff --git a/ml.cpp b/ml.cpp
index 1a17c0d..bfaf636 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -419,7 +419,7 @@ public:
         }
     }
     
-    bool operator!=(Value other) const {
+    bool operator!=(const Value &other) const {
         return !(*this == other);
     }
 
@@ -427,19 +427,19 @@ public:
     /// ORDERING OPERATIONS ////////////////////////////////////////////////////////
     ////////////////////////////////////////////////////////////////////////////////
 
-    bool operator>=(Value other) const {
+    bool operator>=(const Value &other) const {
         return !(*this < other);
     }
     
-    bool operator<=(Value other) const {
+    bool operator<=(const Value &other) const {
         return (*this == other) || (*this < other);
     }
     
-    bool operator>(Value other) const {
+    bool operator>(const Value &other) const {
         return !(*this <= other);
     }
 
-    bool operator<(Value other) const {
+    bool operator<(const Value &other) const {
         // Other type must be a float or an int
         if (other.type != FLOAT && other.type != INT)
             throw Error(*this, Environment(), INVALID_BIN_OP);
@@ -465,7 +465,7 @@ public:
     ////////////////////////////////////////////////////////////////////////////////
 
     // This function adds two lisp values, and returns the lisp value result.
-    Value operator+(Value other) const {
+    Value operator+(const Value &other) const {
         // If the other value's type is the unit type,
         // don't even bother continuing.
         // Unit types consume all arithmetic operations.
@@ -513,7 +513,7 @@ public:
     }
 
     // This function subtracts two lisp values, and returns the lisp value result.
-    Value operator-(Value other) const {
+    Value operator-(const Value &other) const {
         // If the other value's type is the unit type,
         // don't even bother continuing.
         // Unit types consume all arithmetic operations.
@@ -545,7 +545,7 @@ public:
     }
 
     // This function multiplies two lisp values, and returns the lisp value result.
-    Value operator*(Value other) const {
+    Value operator*(const Value &other) const {
         // If the other value's type is the unit type,
         // don't even bother continuing.
         // Unit types consume all arithmetic operations.
@@ -575,7 +575,7 @@ public:
     }
 
     // This function divides two lisp values, and returns the lisp value result.
-    Value operator/(Value other) const {
+    Value operator/(const Value &other) const {
         // If the other value's type is the unit type,
         // don't even bother continuing.
         // Unit types consume all arithmetic operations.
@@ -605,7 +605,7 @@ public:
     }
 
     // This function finds the remainder of two lisp values, and returns the lisp value result.
-    Value operator%(Value other) const {
+    Value operator%(const Value &other) const {
         // If the other value's type is the unit type,
         // don't even bother continuing.
         // Unit types consume all arithmetic operations.