statement -> assignment | cond | loop
assignment -> id := expr
cond -> if boolexpr then statemnt fi |
if boolexpr then statement else statement fi
loop -> while boolexpr do statement od
expr -> boolexpr | numexpr
boolexpr -> numexpr cop numexpr
numexpr -> numexpr + term | term
term -> term * factor | factor
factor -> id | const | (numexpr)