Stepper
-------

Changes for v101:

all steps scroll to bottom automatically.
constants like 'pi' are explicitly expanded in a step.
stepper uses fewer threads internally.

Changes for v102:

Stepper handles intermediate level.
UI redesigned to use "side-by-side" reduction.

Changes for v103:

PRs fixed: 1564, 1277, 1536, 1500, 1561, 1468, 1599, 1631

Changes for v200:

Total rewrite for new syntax. Addition of test suites.
Addition of somewhat more systematic macro unwinding.
Lots of bug fixes.

Changes for v201:

Minor bug fixes.

Changes for v203:

Much more systematic unwinding, intermediate almost ready, redesigned test suite

Changes for v204:

none.

Changes for v205:

v. minor bug fixes.

Changes for v206:

Stepper supports intermediate, minor bug fixes, major rewrite of interface
 between reconstruct and display.

Changes for v206p1:

None.

Changes for v207:

None.

Changes for v208:

minor bug fixes.

Changes for v209:

None.

Changes for v300: 

updated for compatibility.