Redefining a keyword in mit-scheme

gistfile1.txt

MIT/GNU Scheme running under OS X
Type `^C' (control-C) followed by `H' to obtain information about interrupts.

Copyright (C) 2014 Massachusetts Institute of Technology
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  Release 9.2 || Microcode 15.3 || Runtime 15.7 || SF 4.41 || LIAR/C 4.118 || Edwin 3.116
  
1 ]=> (or (quote a) (quote b))

;Value: a

1 ]=> (define or (quote my-or))

;Value: or

1 ]=> or

;Value: my-or

1 ]=> (or (quote a) (quote b))

;The object my-or is not applicable.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify a procedure to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.

2 error>