Linear splines with convenient parameterizations such that:

- coefficients are slopes of consecutive segments
- coefficients capture slope change at consecutive knots

Knot locations can be specified

- manually (
`lspline()`

) - at breaks dividing the range of
`x`

into`q`

equal-frequency intervals (`qlspline()`

) - at breaks dividing the range of
`x`

into`n`

equal-width intervals (`elspline()`

)

Inspired by Stata command `mkspline`

and function `lspline()`

from package **ares** (Junger and Ponce de Leon 2011). As such, the implementation follows Greene (2003), chapter 7.5.2.