func fixNegatives(exp []lex.Token) []lex.Token {
var prev lex.Token
prev.Set(lex.Symbol, "")
for i, a := range exp {
if prev.Type() == lex.Symbol && a.String() == "-" &&
(prev.String() != ")" || prev.String() == "") {
exp[i].Set(lex.Symbol, "i")
}
prev = a
}
return exp
}
