code: def haversin(theta):
      return sin(0.5 * theta) ** 2
  
file_path: bokeh/examples/models/trail.py
imports: from math import atan2, cos, radians, sin, sqrt
  import numpy
  import scipy.ndimage
  from bokeh.document import Document
  from bokeh.embed import file_html
  from bokeh.models import Column, ColumnDataSource, GMapOptions, GMapPlot, Grid, Label, Line, LinearAxis, PanTool, Patches, Plot, Range1d, ResetTool, WheelZoomTool
  from bokeh.sampledata.mtb import obiszow_mtb_xcm
  from bokeh.util.browser import view
label_desc: Use image node_id 1 for calling a global function with example usage: haversin(theta) and returns: unknown