* strip whitespace, leave one space before a keyword on a line, compress any
  other whitespace to a single space
* white is blank, tab, /* ... */
* target-spec line comment, .e.g. #
* symbol = [letter | _ | . | $] [letter | _ | . | $ | digit]*
* stmt: {label}* { directive | instruction | empty }

* %r0..%r15 with %sp == %r15 ? (no, % is also an operator)
* $0..$15 with $sp == $15? (yes, these are just symbols)


* PDP10 syntax:

mnemonic accumulator,address

	movem 1,foo

mnemonic accumulator,

	popj 17,

mnemonic address

	setzm foo

	skipe 0(16)

address prefixed by "@" makes it indirect

	setzm @foo

address suffixed by "(ixreg)" makes it indexed

	setzm 3(16)


";" is line comment char

	foo ;comment

radix is 8 by default; a single-digit number is always decimal
radix can be changed by the RADIX directive
a number can indicate its radix by a ^B, ^O, or ^D prefix (XXX: add ^X for hex)

symbols can use letters, digits, dot, dollar, and percent signs

exp1,,exp2 assembles two 18-bit expressions into a 36-bit value

< expr > parenteses

there is no "semi-colon" like symbol for putting multiple statements
on a single line