NOTE: GLK's approximate ranking of 6 most imporant tagged with
[GLK:1], [GLK:2], ...

========================
SHORT TERM =============  (for curvature-based VR)
========================

IL support for higher-order tensor values (matrices, etc).
    tensor construction [DONE]
    tensor indexing [DONE]
    tensor slicing
    verify that hessians work correctly [DONE]

Outer products [DONE]

Add M dot v, v dot M, and M dot N [DONE]

Identity matrix [DONE]

Zero tensor [DONE]

trace [DONE]

extend norm (|exp|) to tensor[3,3] [DONE]

Matrix addition, subtraction, and scaling  [DONE]

Use ∇⊗ etc. syntax
    syntax [DONE]
    typechecking
    IL and codegen

Add sequence types
    syntax
	types: ty '{' INT '}'
	value construction: '{' e1 ',' … ',' en '}'
	indexing: e '{' e '}'

Infix dot product and cross product [DONE]

lerp on scalars and vectors [DONE]

Infix "^" operator for pow() [DONE]

Code generation support for 1D image data, such as RGBA transfer functions [DONE]

==============================
other SHORT TERM =============  (including needed for LIC)
==============================

test/uninit.diderot:
documents need for better compiler error messages when output variables
are not initialized; the current messages are very cryptic

determinant ("det") for tensor[3,3]

extend norm (|exp|) to all tensor types [DONE for vectors and matrices]

expand trace in mid to low translation

value-numbering optimization

Add type aliases for color types
    rgb = real{3}
    rgba = real{4}

==============================
MEDIUM TERM ================== (including needed for streamlines & tractography)
==============================

[GLK:3] evals & evecs for symmetric tensor[3,3] (requires sequences)

[GLK:4] Save Diderot output to nrrd, instead of "mip.txt"
  For grid of strands, save to similarly-shaped array
  For list of strands, save to long 1-D (or 2-D for non-scalar output) list
  For ragged things (like tractography output), will need to save both
    complete list of values, as well as list of start indices and lengths
    to index into complete list

[GLK:5] Use of Teem's "hest" command-line parser for getting
any input variables that are not defined in the source file

[GLK:6] ability to declare a field in such a way so that probe
positions are *always* clamped to the support of "inside";
there are many cases where this is the sensible behavior.
(More generally, we could also have "repeat" declaration,
copying action of GL_REPEAT in texturing)

ability to emit/track/record variables into dynamically re-sized
runtime buffer

Want: allow X *= Y, X /= Y, X += Y, X -= Y to mean what they do in C,
provided that X*Y, X/Y, X+Y, X-Y are already supported.
Nearly every Diderot program would be simplified by this.

Want: non-trivial field expressions & functions:
  image(2)[2] Vimg = load(...);
  field#0(2)[] Vlen = |Vimg ⊛ bspln3|;
to get a scalar field of vector length, or 
  field#2(2)[] F = Fimg ⊛ bspln3;
  field#0(2)[] Gmag = |∇F|;    
to get a scalar field of gradient magnitude, or 
  field#2(2)[] F = Fimg ⊛ bspln3;
  field#0(2)[] Gmsq = ∇F•∇F;
to get a scalar field of squared gradient magnitude, which is simpler
to differentiate.  However, there is value in having these, even if
the differentiation of them is not supported (hence the indication
of "field#0" for these above)

Want: ability to apply "normalize" to a field itself, e.g.
  field#0(2)[2] V = normalize(Vimg ⊛ ctmr);
so that V(x) = normalize((Vimg ⊛ ctmr)(x)).
Having this would simplify expression of standard LIC method, and
would also help express other vector field expressions that arise
in vector field feature exraction.

tensor fields: convolution on general tensor images

==============================
other MEDIUM TERM ============ (needed for particles)
==============================

run-time birth and death of strands

"initially" supports lists

"initially" supports lists of positions output from 
different initalization Diderot program

spatial data structure that permits strands' queries of neighbors

proper handling of stabilize method

test/vr-kcomp2.diderot: Add support for code like

	(F1 if x else F2)@pos

This will require duplication of the continuation of the conditional
(but we should only duplicate over the live-range of the result of the
conditional.

==============================
other MEDIUM TERM ============
==============================

want: warnings when "D" (reserved for differentiation) is declared as
a variable name (get confusing error messages now)

support for Python interop and GUI

==============================
LONG TERM ==================== 
==============================

Better handling of variables that determines the scope of a variable
based on its actual use, instead of where the user defined it.  So,
for example, we should lift strand-invariant variables to global
scope.  Also prune out useless variables, which should include field
variables after the translation to mid-il.

co- vs contra- index distinction

add ":" for tensor dot product (contracts out two indices
instead of one like •), valid for all pairs of tensors with
at least two indices

some indication of tensor symmetry
(have to identify the group of index permutations that are symmetries)

dot works on all tensors

outer works on all tensors

Einstein summation notation

"tensor comprehension" (like list comprehension)

Python/ctypes interface to run-time

======================
BUGS =================
======================

[GLK:1] test/derivs2.diderot:
test/derivs3.diderot:
gradients & Hessians are not being transformed from index-to-world,
these test programs should produce black images when that is working

[GLK:2] test/fields.diderot:
documents various bugs/questions associated with simple field expressions
(negation, addition, scalar multiplication)

test/read2vecs.diderot:
// HEY (BUG?) shouldn't it be a type error to load this 2-D array of 
// 2-vectors into a 2-D *scalar* field?  Instead, get:
//   uncaught exception Fail [Fail: Error in compiling lic.diderot]
//     raised at driver/main.sml:31.39-31.76
image(2)[] Vimg = load("../data/vorttest.nrrd");

test/vr-kcomp.diderot:
    // HEY (scoping BUG): the strand parameters (e.g. ui, vi) ...

test/zslice2.diderot:
// but is this not valid syntax for creating field in one shot?
//field#2(3)[] F = (load("../data/zimg112.nrrd")) ⊛ bspln3;
// It is valid syntax, but there is a bug in the conversion from HighIL to MidIL

test/zslice2.diderot:
// HEY (bug) bspln5 leads to problems ...
//  uncaught exception Size [size]
//    raised at c-target/c-target.sml:47.15-47.19
//field#4(3)[] F = img ⊛ bspln5;