Expand all leading tabs in markdown files

Given how problematic leading tabs in markdown files can be,
we have expanded all leading tabs in markdown files by:

```sh
find * -type f -name '*.md' -print |
  while read -r file; do
    rm -f "$file.tmp" ;
    expand < "$file" > "$file.tmp" ;
    mv -f -v "$file.tmp" "$file" ;
  done
```

Given the previous changes to `bin/pandoc-wrapper.sh` and other recent
commits and recent pull requests, the were were **NO** changes to
any HTML files.  This is a **good thing** as the expression goes.

No there are no leading tabs in any markdown files.
A check to catch any re-introduction of leading tabs
into any markdown file is on the way.

BTW: We do not have anything against leading tabs in files,
even though we have had to deal with their impact on forming
HTML from markdown files (as well as the calc source code).
It is just that this is one of those cases where tabs conflict
(sometimes caused by people wanting to change the classic
8-space tab model) with a desired outcome.

Perform `make www` to perform the above and test under macOS.

2015/burton/README.md

@@ -193,7 +193,7 @@ shifts mod 32:
 
 ```
     1<<48
-    65536	0x00010000
+    65536       0x00010000
 ```
 
 gcc 4.2.1 without arguments compiles cleanly (e.g. `cc prog.c -o prog`).

2015/burton/road.to.obscurity.md

@@ -17,11 +17,11 @@ were added over time.
 dance began (this is called foreshadowing):
 
 ```
-    ()		forced precedence
-    - ~ ?		unary minus, compl, byteswap
-    * / % & << >>	mul, div, mod, bitand, shift: left, right
-    + - | ^		add, sub, bitor, xor
-    NUM		0-9
+    ()          forced precedence
+    - ~ ?               unary minus, compl, byteswap
+    * / % & << >>       mul, div, mod, bitand, shift: left, right
+    + - | ^             add, sub, bitor, xor
+    NUM         0-9
 ```
 
 This seemed like a reasonable candidate to experiment with obfuscation; perhaps
@@ -34,39 +34,39 @@ The algorithm was and is rather straightforward:
 
 ```
     expr() {
-	    n = term()
-	    t = token()
-	    if (t == ADD) return n += expr()
-	    if (t == SUB) return n -= expr()
-	    if (t == BOR) return n |= expr()
-	    if (t == XOR) return n ^= expr()
-	    ungettoken(t)
-	    return n
+            n = term()
+            t = token()
+            if (t == ADD) return n += expr()
+            if (t == SUB) return n -= expr()
+            if (t == BOR) return n |= expr()
+            if (t == XOR) return n ^= expr()
+            ungettoken(t)
+            return n
     }
     term() {
-	    n = factor()
-	    t = token()
-	    if (t == MUL) return n *= term()
-	    if (t == DIV) return n /= term()
-	    if (t == AND) return n &= term()
-	    if (t == RSH) return n >> term()
-	    if (t == LSH) return n << term()
-	    ungettoken(t)
-	    return n
+            n = factor()
+            t = token()
+            if (t == MUL) return n *= term()
+            if (t == DIV) return n /= term()
+            if (t == AND) return n &= term()
+            if (t == RSH) return n >> term()
+            if (t == LSH) return n << term()
+            ungettoken(t)
+            return n
     }
     factor() {
-	    t = token()
-	    if (t == LPAREN) {
-		    n = expr()
-		    if token() == RPAREN
-			    return n
-		    error()
-	    }
-	    if (t == SUB) return -factor()
-	    if (t == CMP) return ~factor()
-	    if (t == QUE) return byteswap(factor())
-	    if (t == DIG) return strtol()
-	    error()
+            t = token()
+            if (t == LPAREN) {
+                    n = expr()
+                    if token() == RPAREN
+                            return n
+                    error()
+            }
+            if (t == SUB) return -factor()
+            if (t == CMP) return ~factor()
+            if (t == QUE) return byteswap(factor())
+            if (t == DIG) return strtol()
+            error()
     }
 ```
 

2015/burton/spoilers.md

@@ -14,10 +14,10 @@ proceeds from there, using some identities that are confusingly similar and
 interestingly combined (increasing obfuscation):
 
 ```
-    obc	   clr
-    ---	   ---
-    -~x	   x+1
-    ~-x	   x-1
+    obc    clr
+    ---    ---
+    -~x    x+1
+    ~-x    x-1
     y-~x   y+x+1
 ```