Create the REV function from Swap

We have a function REV(“string”,m,n).This function is capable of reversing the caharacters in the string from mth location to nth location. e.g. REV(“abcd”,2,3); the output will be acbd We need to swap a string from a position,e.g. SWAP(“abcdefg”,4)  output needs to be efgabcd.
How can the REV function used do this.

Solution
 ANS : L = string length,N= position given in SWAP function.
SWAP(“abcdefg”,4) = REV(REV(REV(“abcdefg”,N+1,L),1,N),1,L).