code: def sparse_add(vs, x_prev, x_new):
      x_prev = x_prev if x_prev is not None else vs.zeros()
      return x_new.mut_add(x_prev)
  
imports: from itertools import count
  from functools import reduce
  from .tracer import trace, primitive, toposort, Node, Box, isbox, getval
  from .util import func, subval
  import warnings
label_desc: Use image node_id 15 for calling a global function with example usage: sparse_add(vs, x_prev, x_new) and returns: x_new