text: from bifacialvf import simulate  # main program
  from vf import getBackSurfaceIrradiances, getFrontSurfaceIrradiances, getGroundShadeFactors  # main subroutines
  from vf import getSkyConfigurationFactors, trackingBFvaluescalculator, rowSpacing # helper functions
  from sun import hrSolarPos, perezComp, solarPos, sunIncident # solar position and value
  from loadVFresults import loadVFresults # utility for reading result files
  from readepw import readepw
  