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

For RGB color output of 2D transfer function (indexed by
kappa1,kappa2), or for a 1D transfer function, will need
something akin to convolution on vector images

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

Outer products [DONE]

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

Identity matrix [DONE up to code generation]

Zero tensor [DONE up to code generation]

trace [DONE]

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

extend implementation of scaling to work on higher-order tensors (it currently
just works on vectors)

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]

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

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

Decide if we want to allow redefinitions of variables
  (as in vr-lite-cam.diderot)

extend norm (|exp|) to all tensor types

extend normalize to all tensor types

vector fields: convolution on vector images

expand trace in mid to low translation

value-numbering optimization

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

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

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

evals & evecs for symmetric tensor[3,3]

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

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 (and likely I) is declared as a 
variable name (now 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-timez

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

test/mip-cam.diderot:
    // generated C code looks like "(float)p_ui_0--0.5e0f"
    //real rayU = 2.0*camUmax*(real(ui) - -0.5)/real(imgResU) - camUmax;

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;