Re: [eigen] On a flexible API for submatrices, slicing, indexing, masking, etc.

[Yuanchen Zhu <yuanchen.zhu@xxxxxxxxx>]

a + iota * s | until(b) = seq(a, b, s)

BTW we are sort of revisiting the "named parameter" API: 

Iota  == Count

"A+" == Start

"*S" == Incr

"until(b)" == Last

A few key advantages:

- optional argument with default values can just be omitted. 

- no confusion on ordering of parameter.

- the whole _expression_ has a well-defined meaning. It can be seen a special case of indexing with a concrete vector.

- at least for "Count" and "Last", we can omit fixed<N>, and just write iota<Count> and until<Last>.