Error on overriding imported variable

override.scm

(import (rnrs))
(define (+ a b) "blabla")
(+ 1 2)

#|
$ sagittarius -r6 override.scm
Unhandled exception
  Condition components:
  1. &compile
    program: (program (import (rnrs)) (define (+ a b) "blabla") (display
    source: #f
  2. &syntax
    subform: (+)
    form: (define (+ a b) "blabla")
  3. &message attempt to modify immutable variable
  4. &stack-trace

stack trace:
  [1] syntax-error
  [2] pass1/library
  [3] with-error-handler
  [4] dynamic-wind
  [5] eval
  [6] program-r6rs
|#