/ grouped -> list

P:{
 a:{:[@y;(x;y);(x;_f/y)]}
 p:{
  :[~#z                 ;(x;y;z)
    (*z)_in" \t\r\n"    ;(x;y;1_ z)
    "("=*z              ;(.[x;y;,;,()];y,#x . y;1_ z)
    ")"=*z              ;(.[x;-1_ y;,;()];-1_ y;1_ z)
                         (.[x;y;,;1#z];y;1_ z)]}
 a/*(p .)/(();!0;x," ")}

/ list -> applicative

Q:{:[@x;x;"`",,/_f'x]}

/ applicative -> grouped

R:{:["`"=*x;_f[*d],{:[1=#x;x;"(",x,")"]}_f[*|d:D 1_ x];x]}
D:{(0,1+(+\-1 1 x="`")?-1)_ x}

/ lambda elimination:

L:{|*(M .)/(();|x)}
M:{:[~#y;(x;y);"."=*y;(|N[y 1;|x];2_ y);(x,*y;1_ y)]}
N:{,/(O[x].)/(();y)}
O:{(:[~#z;y;"`"~*z;y,"``S";x~*z;y,"I";y,"`K",*z];1_ z)}

Y:+(("``S`KxI"          ;,"x")
    ("``S`Kx`Ky"        ;"`K`xy")
    ("``S`Kxy"          ;"``Bxy")
    ("``Sx`Ky"          ;"``Cxy"))

t:L"x.y.``-xy"

Y:(P R@)''Y
t:P R t
y:("Y";(("S";"K+");"K0"))
d:{{{e[y;z;x]}/[x;*Y;Y 1]}/x}
e:{:[@z;z;_n~d:f[.();x;z:_f[x;y]'z];z;g[d;y]]}
f:{:[_n~x;x;~@y;:[~@z;_f/[x;y;z]];y _in _ci 97+!26;@[x;`$y;:;z];@y;:[y~z;x]]}
g:{:[4:y;{:[1=#x;*x;x]}@h/[y,"";,/$!x;x[]];_f[x]'y]}
h:{@[x;&x~\:y;:[;z]]}
d t