code: def dsi(si_s, si_t):
      dsi = si_t - si_s
      return dsi
  
imports: import pytest
  import numpy
  import pandas
  from pandas.testing import assert_frame_equal
  from deepgraph import DeepGraph
label_desc: Use image node_id 1 for calling a global function with example usage: dsi(si_s, si_t) and returns: dsi