Update BASIC.ipynb

ipynb/BASIC.ipynb

@@ -84,7 +84,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The only complicated part is the syntax for numbers: optional digits followed by an optional decimal point, some digits, and optionally a power of 10 marked by `\"E\"` and followed by an (optional) minus sign and some digits. \n",
+    "In this version of BASIC, variable names must be either a single letter, or a letter followed by a digit. The only complicated part is the syntax for numbers: optional digits followed by an optional decimal point, some digits, and optionally a power of 10 marked by `\"E\"` and followed by an (optional) minus sign and some digits. \n",
     "Example usage of `tokenize`:"
    ]
   },
@@ -201,10 +201,6 @@
     "    top = peek()\n",
     "    if constraint is None or (top == constraint) or (callable(constraint) and constraint(top)):\n",
     "        return tokens.pop(0)\n",
-    "    \n",
-    "def remove_spaces(line): \n",
-    "    \"Remove white space from line, except space inside double quotes.\"\n",
-    "    return \n",
     "\n",
     "def lines(text): \n",
     "    \"A list of the non-empty lines in a text.\"\n",
@@ -396,7 +392,6 @@
     "    'RETURN': [],\n",
     "    'DIM':    [list_of(variable)], \n",
     "    'REM':    [anycharacters],\n",
-    "    'A':    []\n",
     "    }"
    ]
   },
@@ -699,7 +694,7 @@
    "source": [
     "# Parsing: Grammar of Expressions\n",
     "\n",
-    "Now for the single most complicated part of the grammar: the `expression`.  The biggest complication is operator precedence: the string `\"A + B * X + C\"` should be parsed as if it were `\"A + (B * X) + C\"`, and not as `\"(A + B) * (X + C),\"` because multiplication binds more tightly than addition. There are [many schemes](https://en.wikipedia.org/wiki/Operator-precedence_parser) for parsing expressions, I'll use  [an approach](https://groups.google.com/forum/#!topic/comp.compilers/ruJLlQTVJ8o) attributed to Keith Clarke.\n",
+    "Now for the single most complicated part of the grammar: the `expression`.  The biggest complication is operator precedence: the string `\"A + B * X + C\"` should be parsed as if it were `\"A + (B * X) + C\"`, and not as `\"(A + B) * (X + C),\"` because multiplication binds more tightly than addition. There are [many schemes](https://en.wikipedia.org/wiki/Operator-precedence_parser) for parsing expressions, I'll use  [an approach](https://groups.google.com/forum/#!topic/comp.compilers/ruJLlQTVJ8o) attributed to Keith Clarke called Compact Recursive-Descent Parsing.\n",
     "\n",
     "Like all our grammatical categories, calling `expression()` pops off some tokens and returns a data object. The first thing it does is parse one of five types of simple \"primary\" expressions: \n",
     "a number like `1.23`; \n",