Function eq_ind_truncate_low_inplace 

pub fn eq_ind_truncate_low_inplace<Cube: Hypercube, P: PackedField, Data: BufferData<P>>(
    values: &mut FieldBuffer<P, Data>,
    truncated_log_len: usize,
)

Truncates a built equality indicator expansion to its low indexed variables.

Each step contracts the two halves of the buffer, stripping the highest variable. That removes the highest variable’s basis factor, whatever its coordinate was. Truncating to n' variables therefore leaves the indicator over r_0, ..., r_{n'-1}.

The expansion occupies a prefix of the buffer. Scalars after the truncated length are dropped.

Preconditions

• the truncated length must be at most the buffer’s current length