code: def _find_interval(self, v):
      m = len(self.intervals)
      i = 0
      while i < m - 1 and self.intervals[i] <= v:
          i += 1
      return i
  
imports: from sympy.core.basic import Basic
  from sympy.core.symbol import Symbol, symbols
  from sympy.utilities.lambdify import lambdify
  from .util import interpolate, rinterpolate, create_bounds, update_bounds
  from sympy.utilities.iterables import sift
label_desc: Use image node_id 3 for calling the ColorGradient obj's underlying member method code with example usage: obj._find_interval(v) and returns: i