Type checker allows a zero literal index for sequences

[nickbattle]

VDM uses 1-relative sequence indexes, but it is a common mistake for people to write seq(0) when they mean seq(1). This should give an error in the type checker; currently the type checking is clean and it fails at runtime. So for example:

values
	S = [1,2,3,4];
	T = S(0);

That produces no TC errors, but at runtime (initialization here) you get "Error 4058: Value 0 is not a nat1".

[nickbattle]

Now fixed in ncb/development:

Parsed 1 module in 0.103 secs. No syntax errors
Error 3056: Sequence application argument must > 0 in 'DEFAULT' (test.vdm) at line 3:9
Type checked 1 module in 0.146 secs. Found 1 type error
Bye