If you can read this then you can understand the code, as every symbol is a
function in the source. Of course S is the starting symbol.


S -> E

E -> F
E -> E '+' F
E -> E '-' F

F -> G
F -> F '*' G
F -> F '/' G
F -> F '%' G

G -> H
G -> H '^' G

H -> term
H -> term '!'

term -> left_par E right_par
term -> constant
term -> function

left_par  -> '('
left_par  -> '['

right_par -> ')'
right_par -> ']'