diff --git a/kiwi.lua b/kiwi.lua
index 31638e2..7c03db2 100644
--- a/kiwi.lua
+++ b/kiwi.lua
@@ -96,6 +96,8 @@ struct KiwiVar {
 
 void kiwi_var_free(KiwiVar* var);
 
+void kiwi_expression_add_term(const KiwiExpression* expr, KiwiVar* var, double coeff, KiwiExpression* out);
+void kiwi_expression_set_constant(const KiwiExpression* expr, double constant, KiwiExpression* out);
 ]])
 else
    ffi.cdef([[
@@ -296,20 +298,29 @@ else
    end
 end
 
----@param expr kiwi.Expression
----@param var kiwi.Var
----@param coeff number?
----@nodiscard
-local function add_expr_term(expr, var, coeff)
-   local ret = ffi_new(Expression, expr.term_count + 1) --[[@as kiwi.Expression]]
-   ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
-   local dt = ret.terms_[expr.term_count]
-   dt.var = var
-   dt.coefficient = coeff or 1.0
-   ret.constant = expr.constant
-   ret.term_count = expr.term_count + 1
-   ljkiwi.kiwi_expression_retain(ret)
-   return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+local add_expr_term
+if RUST then
+   ---@param expr kiwi.Expression
+   ---@param var kiwi.Var
+   ---@param coeff number?
+   ---@nodiscard
+   function add_expr_term(expr, var, coeff)
+      local ret = ffi_new(Expression, expr.term_count + 1)
+      ljkiwi.kiwi_expression_add_term(expr, var, coeff or 1.0, ret)
+      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+   end
+else
+   function add_expr_term(expr, var, coeff)
+      local ret = ffi_new(Expression, expr.term_count + 1) --[[@as kiwi.Expression]]
+      ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
+      local dt = ret.terms_[expr.term_count]
+      dt.var = var
+      dt.coefficient = coeff or 1.0
+      ret.constant = expr.constant
+      ret.term_count = expr.term_count + 1
+      ljkiwi.kiwi_expression_retain(ret)
+      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+   end
 end
 
 ---@param constant number
@@ -719,34 +730,33 @@ do
       local b_count = b.term_count
       local ret = ffi_new(Expression, a_count + b_count) --[[@as kiwi.Expression]]
 
-      for i = 0, a_count - 1 do
-         local dt = ret.terms_[i] --[[@as kiwi.Term]]
-         local st = a.terms_[i] --[[@as kiwi.Term]]
-         dt.var = st.var
-         dt.coefficient = st.coefficient
-      end
-      for i = 0, b_count - 1 do
-         local dt = ret.terms_[a_count + i] --[[@as kiwi.Term]]
-         local st = b.terms_[i] --[[@as kiwi.Term]]
-         dt.var = st.var
-         dt.coefficient = st.coefficient
-      end
+      ffi_copy(ret.terms_, a.terms_, SIZEOF_TERM * a_count)
+      ffi_copy(ret.terms_ + a_count, b.terms_, SIZEOF_TERM * b_count)
       ret.constant = a.constant + b.constant
       ret.term_count = a_count + b_count
       ljkiwi.kiwi_expression_retain(ret)
       return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
    end
 
-   ---@param expr kiwi.Expression
-   ---@param constant number
-   ---@nodiscard
-   local function new_expr_constant(expr, constant)
-      local ret = ffi_new(Expression, expr.term_count) --[[@as kiwi.Expression]]
-      ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
-      ret.constant = constant
-      ret.term_count = expr.term_count
-      ljkiwi.kiwi_expression_retain(ret)
-      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+   local new_expr_constant
+   if RUST then
+      ---@param expr kiwi.Expression
+      ---@param constant number
+      ---@nodiscard
+      function new_expr_constant(expr, constant)
+         local ret = ffi_new(Expression, expr.term_count)
+         ljkiwi.kiwi_expression_set_constant(expr, constant, ret)
+         return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+      end
+   else
+      function new_expr_constant(expr, constant)
+         local ret = ffi_new(Expression, expr.term_count) --[[@as kiwi.Expression]]
+         ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
+         ret.constant = constant
+         ret.term_count = expr.term_count
+         ljkiwi.kiwi_expression_retain(ret)
+         return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
+      end
    end
 
    ---@return number
diff --git a/rjkiwi/src/expr.rs b/rjkiwi/src/expr.rs
index 440ae75..839b524 100644
--- a/rjkiwi/src/expr.rs
+++ b/rjkiwi/src/expr.rs
@@ -48,6 +48,61 @@ impl KiwiExpression {
     }
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn kiwi_expression_add_term(
+    e: *const KiwiExpressionPtr,
+    v: *const KiwiVar,
+    coefficient: c_double,
+    out: *mut KiwiExpressionPtr,
+) {
+    let Some(e) = KiwiExpression::try_from_raw(e) else {
+        return;
+    };
+
+    let n_terms = (e.terms_.len() + 1).min(c_int::MAX as usize);
+
+    let out = core::slice::from_raw_parts_mut(out as *mut (), n_terms) as *mut [()]
+        as *mut KiwiExpression;
+
+    (*out).owner = out as *mut c_void;
+    (*out).term_count = n_terms as c_int;
+    (*out).constant = e.constant;
+
+    for (o, i) in (*out).terms_.iter_mut().zip(e.terms_.iter()) {
+        o.var = KiwiVar::retain_raw(i.var);
+        o.coefficient = i.coefficient;
+    }
+    (*out).terms_[n_terms - 1] = KiwiTerm {
+        var: KiwiVar::retain_raw(v),
+        coefficient,
+    };
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn kiwi_expression_set_constant(
+    e: *const KiwiExpressionPtr,
+    constant: c_double,
+    out: *mut KiwiExpressionPtr,
+) {
+    let Some(e) = KiwiExpression::try_from_raw(e) else {
+        return;
+    };
+
+    let n_terms = e.terms_.len();
+
+    let out = core::slice::from_raw_parts_mut(out as *mut (), n_terms) as *mut [()]
+        as *mut KiwiExpression;
+
+    (*out).owner = out as *mut c_void;
+    (*out).term_count = n_terms as c_int;
+    (*out).constant = constant;
+
+    for (o, i) in (*out).terms_.iter_mut().zip(e.terms_.iter()) {
+        o.var = KiwiVar::retain_raw(i.var);
+        o.coefficient = i.coefficient;
+    }
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn kiwi_expression_retain(e: *mut KiwiExpressionPtr) {
     let Some(e) = not_null_mut(e) else {