added abstract expression

src/visualising/expression/library/alphabet/l_G.txt

src/visualising/expression/library/alphabet/l_H.txt

src/visualising/expression/library/alphabet/l_I.txt

src/visualising/expression/library/alphabet/l_J.txt

src/visualising/expression/library/alphabet/l_L.txt

src/visualising/expression/library/alphabet/l_M.txt

src/visualising/expression/library/alphabet/l_N.txt

src/visualising/expression/library/alphabet/l_O.txt

src/visualising/expression/library/alphabet/l_P.txt

src/visualising/expression/library/alphabet/l_R.txt

src/visualising/expression/library/alphabet/l_S.txt

src/visualising/expression/library/alphabet/l_T.txt

src/visualising/expression/library/alphabet/l_U.txt

src/visualising/expression/library/alphabet/r_A.txt

src/visualising/expression/library/alphabet/r_B.txt

src/visualising/expression/library/alphabet/r_C.txt

src/visualising/expression/library/alphabet/r_D.txt

src/visualising/expression/library/alphabet/r_E.txt

src/visualising/expression/library/alphabet/r_G.txt

src/visualising/expression/library/alphabet/r_H.txt