sigma= { +, -, *, /, ^, (, ), numero, ;  }

linea::= expr ;

expr::=  term B | + term B | - term B
B::= +term B | -term B | <lambda>

term:: factor C
C:= *factor | /factor | <lambda>

factor::= prim E
E::= ^factor | <lambda>

prim::= numero | (expr)
========================================

U:= term | + term | - term
expr::= U { + term | - term } 

term ::= factor { * factor | / factor }

factor::= prim { ^factor}

prim::= numero | (expr)