remove the misleading "range check helper" API

src/lib/field.ts

@@ -1007,35 +1007,6 @@ class Field {
       .seal();
   }
 
-  /**
-   * Create a new {@link Field} element from the first `length` bits of this {@link Field} element.
-   *
-   * The `length` has to be a multiple of 16, and has to be between 0 and 255, otherwise the method throws.
-   *
-   * As {@link Field} elements are represented using [little endian binary representation](https://en.wikipedia.org/wiki/Endianness),
-   * the resulting {@link Field} element will equal the original one if it fits in `length` bits.
-   *
-   * @param length - The number of bits to take from this {@link Field} element.
-   *
-   * @return A {@link Field} element that is equal to the `length` of this {@link Field} element.
-   */
-  rangeCheckHelper(length: number) {
-    checkBitLength('Field.rangeCheckHelper()', length);
-    if (length % 16 !== 0)
-      throw Error(
-        'Field.rangeCheckHelper(): `length` has to be a multiple of 16.'
-      );
-    let lengthDiv16 = length / 16;
-    if (this.isConstant()) {
-      let bits = Fp.toBits(this.toBigInt())
-        .slice(0, length)
-        .concat(Array(Fp.sizeInBits - length).fill(false));
-      return new Field(Fp.fromBits(bits));
-    }
-    let x = Snarky.field.truncateToBits16(lengthDiv16, this.value);
-    return new Field(x);
-  }
-
   /**
    * **Warning**: This function is mainly for internal use. Normally it is not intended to be used by a zkApp developer.
    *

src/lib/gadgets/range-check.ts

@@ -11,7 +11,6 @@ export {
   rangeCheck32,
   multiRangeCheck,
   compactMultiRangeCheck,
-  rangeCheckHelper,
   rangeCheckN,
   isInRangeN,
   rangeCheck8,
@@ -239,6 +238,12 @@ function rangeCheck1Helper(inputs: {
 
 /**
  * Helper function that creates a new {@link Field} element from the first `length` bits of this {@link Field} element.
+ *
+ * This returns the `x` truncated to `length` bits. However, it does **not** prove this truncation or any
+ * other relation of the output with `x`.
+ *
+ * This only proves that the output value is in the range [0, 2^length), and so can be combined
+ * with the initial value to prove a range check.
  */
 function rangeCheckHelper(length: number, x: Field) {
   assert(