Function eq_ind_truncate_low_inplace 

pub fn eq_ind_truncate_low_inplace<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 sums the two halves of the buffer, stripping the highest variable. Truncating to n' variables 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