f(x, y, z)  // traditional function call notation

(f x y z)   // Lisp function call notation

       f
     / | \
    x  y  z


f()  // traditional function call with no arguments

(f)  // Lisp notation for a function call with no arguments, S-expression

f    // but as a tree expression, it looks like a variable, not a function

(apply f)         // Language_7 function call with no arguments notation

(appply f x y z)  // Language_7 function call notation


f(x) = x*x
g(x) = x*x*x
y = if b then f(x) else g(x)
h = if b then f else g        // h is a function, either f or g
y = h(x)                      // call function h

y = (if b then f else g)(x)   // call either f or g
y = (b ? f : g)(x)            // call either f or g
(var y ((if b f g) x))        // Lisp, S-expression (this is not a tree!)
(var y (apply (if b f g) x)   // Language_7 tree expression


(a b c)         // S-expression (which is a tree), call a with arguments b and c
(apply a b c)   // tree expression, call a with arguments b and c

((a b) c))               // S-expression (not a tree!)
(apply (apply a b) c)    // Language_7 tree expression

(((a b) c) d e)                    // S-expression (not a tree!)
(apply (apply (apply a b) c) d e)  // Language_7 tree expression


f(x) = x*x*x     f = \x -> x*x*x
f(z) = z*z*z     f = \z -> z*z*z
f(3)
(\z -> z*z*z)(3)             // calling an anonymous lambda expression
((lambda x x*x*x) 3)         // Lisp, S-expression
(apply (lambda x x*x*x) 3)   // Languge_8 tree