code: LocalVariableStack addVariable(String name, PyExpr varExpression) {
      Preconditions.checkState(!localVarExprs.isEmpty());
      localVarExprs.peek().put(name, varExpression);
      return this;
    }
docs: Adds a variable to the current reference frame.
  
  @param name The name of the variable as used by calling expressions.
  @param varExpression The underlying expression used to access the variable.
  @return A reference to this object.