Using if_else.

std/constraints.asm

@@ -9,3 +9,11 @@ let make_conditional: Constr, expr -> Constr = |constraint, condition| match con
     Constr::Permutation((Option::Some(sel_l), sel_r), exprs) => Constr::Permutation((Option::Some(sel_l * condition), sel_r), exprs),
     Constr::Connection(_) => std::check::panic("Connection constraints cannot be conditional"),
 };
+
+/// Either one constraint or the other, depending on a boolen condition.
+let if_else: expr, Constr, Constr -> Constr = |condition, if_true, if_false| match (if_true, if_false) {
+    (Constr::Identity(l_t, r_t), Constr::Identity(l_f, r_f)) =>
+        condition * (l_t - r_t) +
+        (1 - condition) * (l_f - r_f) = 0,
+    _ => std::check::panic("if_else can only be used with two identity constraints"),
+};

std/machines/small_field/pointer_arith.asm

@@ -25,10 +25,9 @@ let word_increment_ptr: expr, expr, expr, expr -> Constr[] = constr |pre_high, p
 
     // Increment polynomials, to be used by the caller in constraints:
     [
-        // If low limb is about to overflow, next value must be 0:
-        low_overflow * post_low +
-        // Otherwise, next value is current plus 4:
-        (1 - low_overflow) * (pre_low + 4 - post_low) = 0,
+        // If low limb is about to overflow, next value must be 0,
+        // otherwise, next value is current plus 4.
+        std::constraints::if_else(low_overflow, post_low = 0, post_low = pre_low + 4),
 
         // Set high limb, incremented if low overflowed:
         post_high - pre_high - low_overflow = 0