Part 7
More Properties of the Part_btwn()

4. Concatenating two strings α and β produces a string, such that, the removal of the first part of the string equal in length to the first string produces the first string, and removal of the second part of the string equal in length to the second string produces that second string.

Prt_btwn (0, | α |, α o β ) = α and
Prt_btwn (| α |, | α o β |, α o β) = β;

Example:
α = <1, 4 > and β = < 7, 3, 8>;
| α | = 2 and | β | = 3;
α o β = <1, 4, 7, 3, 8> and | α o β | = 5; then
Prt_btwn (0, 2, <1, 4, 7, 3, 8>) = <1, 4>, which is = α;
Prt_Btwn (2, 5, <1, 4, 7, 3, 8>) = <7, 3, 8>, which is = β;