When writing your typechecker you need to construct visitor that returns a 
String.  In order to facilitate the generation of code later, I recommend that
you also store the actual type inside each abstract syntax node.  I did this
by adding the following field to baseinfo, which is inherited by every node:

    String mjtype = "void";   // "void" is just the convenient default type

Now, whatever type you decide to return after a visit, you should also set
this variable to the same value.  For example:

    public String visit(lessthanexp x)
    {
	String t1 = x.e1.accept(this);
	String t2 = x.e2.accept(this);
	if (!t1.equals("int") || !t2.equals("int"))
	    report("malformed arithmetic expression",x.line);	
	x.mjtype = "boolean";
	return "boolean";
    }

Now when you compile a callexp, you nolonger have to lookup the symbol table,
all you have to say is:

  String classname = x.e1.mjtype;