class_docstr: A reference frame in classical mechanics.
  
  ReferenceFrame is a class used to represent a reference frame in classical
  mechanics. It has a standard basis of three unit vectors in the frame's
  x, y, and z directions.
  
  It also can have a rotation relative to a parent frame; this rotation is
  defined by a direction cosine matrix relating this frame's basis vectors to
  the parent frame's basis vectors.  It can also have an angular velocity
  vector, defined in another frame.
docstr: Returns basis vector for the provided index, if the index is a string.
  
  If the index is a number, returns the coordinate variable correspon-
  -ding to that index.
docstr_tok: ["Returns","basis","vector","for","the","provided","index",",","if","the","index","is","a","string",".","If","the","index","is","a","number",",","returns","the","coordinate","variable","correspon-","-ding","to","that","index","."]
code: def __getitem__(self, ind):
      """
      Returns basis vector for the provided index, if the index is a string.
  
      If the index is a number, returns the coordinate variable correspon-
      -ding to that index.
      """
      if not isinstance(ind, str):
          if ind < 3:
              return self.varlist[ind]
          else:
              raise ValueError("Invalid index provided")
      if self.indices[0] == ind:
          return self.x
      if self.indices[1] == ind:
          return self.y
      if self.indices[2] == ind:
          return self.z
      else:
          raise ValueError("Not a defined index")
  
imports: from sympy import diff, expand, sin, cos, sympify, eye, zeros, ImmutableMatrix, MatrixBase
  from sympy.core.symbol import Symbol
  from sympy.simplify.trigsimp import trigsimp
  from sympy.physics.vector.vector import Vector, _check_vector
  from sympy.utilities.misc import translate
  from warnings import warn
label_desc: Use image node_id 2 for calling the ReferenceFrame obj's underlying member method code with example usage: obj.__getitem__(ind) and returns: self, self, self, self