public void Substitute(string variable, string expr)
{
var exptokens = new Lexer("(" + expr + ")").GenerateTokens();
int i = 0;
while (i < tokens.Count)
{
if (tokens[i].type == Token.Type.VARIABLE && tokens[i].value == variable)
{
tokens.RemoveAt(i);
tokens.InsertRange(i, exptokens);
i += exptokens.Count;
}
i++;
}
}