code: def _zero_triband(a, lower=0):
      """
      Explicitly zero out unused elements of a real symmetric banded matrix.
  
      INPUTS:
         a   -- a real symmetric banded matrix (either upper or lower hald)
         lower   -- if True, a is assumed to be the lower half
      """
  
      nrow, ncol = a.shape
      if lower:
          for i in range(nrow):
              a[i, (ncol - i) :] = 0.0
      else:
          for i in range(nrow):
              a[i, 0:i] = 0.0
      return a
  
imports: import numpy
  import numpy.linalg
  from scipy.linalg import solveh_banded
  from scipy.optimize import golden
  from models import _hbspline
  import warnings
label_desc: Use image node_id 6 for calling a global function with example usage: _zero_triband(a, lower) and returns: a