This Challenge is derived from GJam March 2016 Annual I/O for Password Security. This is the small-1 case of only a single password
The GJam story goes that a random block set A:Z exists to the child of a paranoid corporate president. He is worried that his password(s) may exist in the block pattern. Produce a 26 character block sequence that does not contain his password. If no sequence can be made that does not contain his strong password output 'IMPOSSIBLE'.
Input: [PW], string of 1 to 26 characters
Output: [Pstr], string containing A:Z with no instance of PW or 'IMPOSSIBLE'
Examples: [PW] [Pstr]
[X] [IMPOSSIBLE] [QQ][ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Theory: Single password case is a three liner.