[texhax] help with identifying some macros

[Barbara Beeton]

    \strut, \vtop and lblot. I'd be grateful for a brief description of each,
    more precisely, the general effect on the presentation.

\strut gives a vertical rule of zero width
and the height and depth of a parenthesis.
its purpose is to ensure even line spacing,
especially in alignments like the one you
cite.

\vtop ensures that the first line of the
aligned block is on the same baseline as
the preceding text, "birthday".

the hash # is where an argument is dropped
into a frame.  (it's equivalent to #1 for
the first argument in a \def.)  so the
first line expands to
  John \mapsto 25--Mar

hope that makes sense.  this is plain tex,
*very* plain tex.
						-- bb

    Here's an example of two of them in use taken from The Z Notation: a
    Reference Manual by Michael Spivey.
    
    $birthday$:
    \[
    	known = \{\,{\rm John, Mike, Susan}\,\} \\
    \also
    	birthday = \{\,\vtop{\halign{\strut#\hfil&${}\mapsto{}$#\hfil\cr
    			John&  25--Mar,\cr
    			Mike&  20--Dec,\cr
    			Susan& 20--Dec\,\}.\cr}}
    \]
    Truth be told I've seldom come across anything so cryptic. For example,
    what does the "#" signify?
    Any help would be most appreciated.
    Thanks in advance
    Paul