Substrings

We implement substrings of strings in the following way. Consider the following sequence of commands:

BD>string : A=abcdefghi

BD>string : B=tuv[A 3,6]wxyz

The first command defines a string whose name is `A'. The second command defines a second string, called `B', whose definition part includes a substring extracted from the string whose name is `A'. Indeed, the syntax ` ' means that we should replace ` ' by the third, fourth, fifth, and sixth characters of the string `A'. Thus, `B' will be defined as `tuvcdefwxyz'.

In general, if is detected as input, then `name' is assumed to be the name of a string and a and b to be integers (or equations which will be rounded to integers) marking the desired substring of this named string. The to the characters inclusive of this named string then replace the piece of input in square brackets. The square brackets are also removed.

The rules governing substrings are as follows. Suppose that we encounter the sequence of characters in a command line. N should be the name of a string and a and b are equations which will be rounded to the nearest integer. Suppose that L is the length of the string whose name is N.

• is the substring formed by the characters of N whenever
• if a=0 then a=1 is assumed, and a warning is given.
• if b>L then b=L is assumed, and a warning is given.
• if a<0 then a=L-b+1. This gives us a way of referring to the final b characters of N.
• if a>b then the substring is empty, so that the construction is simply removed.