code: func interpolationFuncLog() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeFloat, ast.TypeFloat},
  		ReturnType: ast.TypeFloat,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return math.Log(args[0].(float64)) / math.Log(args[1].(float64)), nil
  		},
  	}
  }
docs: // interpolationFuncLog returns the logarithnm.