class StringInput

A StringInput object is constructed with a multi-line string and when the get() method is called it returns each of those lines in turn, returning undef when EOF is reached.

In addition:

- if you override the method `mapLine()` each line will
    be passed to that method, which can convert the line
    to any JavaScript value, and can fetch additional
    lines, using the `fetch()` method, in the process.

- the line `#include <filename>` (which may include preceding
    indentation) will result in the given file being read and
    used for input until the file is exhausted:

    1. <filename> must be a simple filename (no path allowed)
    2. File is searched for in this order:
        a. the same directory as the file containing #include
        b. these directories, if defined in hPrivEnv for these extensions:
            '.md':   'DIR_MARKDOWN',
            '.taml': 'DIR_DATA',
            '.txt':  'DIR_DATA',

    2. The option hIncludePaths was passed to the StringInput
        constructor
    3. The file extension of the <filename> was included as
        a key in hIncludePaths (the key must start with '.')

    The indentation of the line containing '#include' will be
    added to each line of the file being included.

Environment variables to control include paths:

DIR_ROOT DIR_MARKDOWN DIR_DATA DIR_TEXT

Parsing a PLL (Python-like language)

1. Define a mapping function
2. Use function treeFromBlock to get the tree

Let's say that you want to create a language for creating "expression objects" that allows functions:

add - to add some numbers
subtract - to add some numbers
multiply - to add some numbers
divide - to add some numbers
sigma - to sum numbers over some index set

For example, these are valid expressions:

sum
    13
    49
    53

multiply
    sum
        13
        22
        53
    22

Numbers must be integers. We also want to allow identifiers, which must be single upper-case letters:

multiply
    sum
        13
        X
    divide
        Y
        3

Here is a valid sigma expression:

sigma
    I in range(5)
    multiply
        I
        22

i.e. sigma expects 2 parts:

1. <identifier> in range(<number> or <identifier>)
2. an expression

SYNOPSIS:

mapper = (str) ->
    if lMatches =