letter ::= a | b | c | ....

digit ::= 1 | 2 | 3 | ...

var ::= letter | {letter | digit}

num ::= {digit} | {digit} . {digit}

// the following defines the precedence, according to https://en.wikipedia.org/wiki/Order_of_operations

op2 ::= + | -

op3 ::= * | /

op4 ::= + | -

op6 ::= < | <= | > |>=

op7 ::= == | !=

op13 ::= ? | :

op14 ::= = |

a = 1 * 2 + 3 < 4 == 5 ? 6 : 7

<e3->7

<--e4--->

<----e6----->

<-------e7------->

<----------e13----------->

<-----------e14-------------->

//

expr0 ::= var | num | expr1 ex:abc, 13, a2, (...)

expr1 ::= ( expr13 )

expr2 ::= expr0 | op2 expr0 ex:+abc, -4, 5

expr3 ::= expr2 | expr2 op3 expr3 ex:a*4, -2*5, 2*-5, -1*(...)

expr4 ::= expr3 | expr3 op4 expr4 ex:a*4 + 3, a*4 + (...)

expr6 ::= expr4 | expr4 op6 expr6 ex:a*4 + 3 < 4

expr7 ::= expr6 | expr6 op7 expr7 ex:a*4 + 3 < 4 != 7

expr13 ::= expr7 | expr7 ? expr7 : exp7

expr14 ::= var op14 expr13

expr ::= expr14 | expr13