# Implement an interpreter for simple ‘while programs’ (so called
# because their only looping construct is ‹while›). The syntax is as
# follows:
#
#  • one line = one statement (no exceptions),
#  • the program is a sequence of statements,
#  • blocks are delimited by indentation (1–5 spaces),
#  • there are following statement types:
#    ◦ assignment,
#    ◦ ‹if› statement,
#    ◦ ‹while› statement.
#
# All variables are global and do not need to be declared (they come
# into existence when they are first used, with a default value 0).
# Variables are always integers. Variable names start with a letter
# and may contain letters, underscores and digits.
#
# The ‹if› and ‹while› statements are followed by a «body»: a block
# indented one space beyond the ‹if› or ‹while› itself. The body
# might be empty. The ‹if› and ‹while› keywords are followed by a
# single variable name. Zero means false, anything else means true.
#
# Assignments are of two forms:
#
#  • constant assignments of the form ‹name = number› (where
#    ‹number› is an integer written in decimal, and might be
#    negative),
#  • 3-address code operations, of the form
#    
#        name₀ = operation name₁ name₂
#
# Valid operations are:
#
#  • logic: ‹and›, ‹or›, ‹nand› (the result is always 0 or 1),
#  • arithmetic: ‹add›, ‹sub›, ‹mul›, ‹div›,
#  • relational:
#    ◦ ‹lt›, ‹gt› (less/greater than),
#    ◦ ‹eq› (equals),
#    ◦ ‹leq› and ‹geq› (less/greater or equal).
#
# Example program:
#
#     x = 0
#     y = 7
#     one = 1
#     if x
#      x = add x x
#     while y
#      y = sub y one
#      x = add x one
#
# Write a function ‹do_while› which takes a ‘while program’ (as a
# string) and returns a dictionary with variable names as keys and
# their final values as values (of type ‹int›).
#
# If the program contains an error, create a special variable named
# ‹#error› and set its value to the offending line number. Return
# immediately after encountering the error. In this case, other
# variables may or may not be included in the resulting dictionary.
#
# Check syntax before you start executing the program (i.e. the
# following program should return an error on line 3 and should
# «not» loop forever):
#
#     x = 1
#     while x
#     x ++
#
# Syntax errors may be due to malformed statements (e.g. ‹while x =
# 1›, ‹x ++› above, etc.), or due to undefined operations (e.g. ‹x =
# mod x y›). Report the first error (nearest to the top of the
# input). At runtime, detect and report any attempts to divide by
# zero.