mid-measure \time change

[Dan Eble]

I think this algorithm would make all of the following examples work
without \partial.

1. Convert the duration remaining in the measure to an integer number
   of beats in the old meter (B) + remainder (R).

2. Convert the number of beats to a duration in the new meter and add
   R to yield a total duration (D).

3. If D is less than a full measure in the new meter, set the position
   to D before the next bar line.  Don't warn; if this is not what the
   user expected, a bar check warning is likely anyway.

4. Otherwise, warn.
-- 
Dan

\version "2.19.0"
\language "english"

% Known problems:
%   warning: \time in mid-measure at ...
%  After the break the bar number is not parenthesized.

\layout {
  indent = 0
  ragged-right = ##t
  
  \context {
    \Score
    barNumberVisibility = #all-bar-numbers-visible
    \override BarNumber.break-visibility = #end-of-line-invisible
  }
}

% Change from triple to quadruple meter.
% Beat note duration remains the same; meter remains simple.
% Anacrusis completes the measure in the previous time signature.
\score {
  \new Staff \relative b' {
    s1*0^"Es waren Hirten zu Bethlehem (Palmer)"
    \key f \major
    
    \time 3/4
    c8( b d c) bf( g) | f2 \bar "||" \break

    \time 4/4
    \partial 4
    f4 | f8 f f4 a8 a a a |
  }
}

% Same as above with compound meter.
\score {
  \new Staff \relative b' {
    s1*0^"My Redeemer (McGranahan)"
    \key af \major

    \time 9/8
    bf4.~ bf4 bf8 bf( af) g | af4.~ af \bar "||" \break
    
    \time 12/8
    \partial 4.
    af8( bf) c | df2.~ df4 df8 f,( af) df |
  }
}

% Meter remains quadruple.
% Beat note duration changes with change from simple to compound
% meter.
% Anacrusis completes the measure in terms of beats, but not in terms
% of duration.
\score {
  \new Staff \relative b' {
    s1*0^"Let Us Crown Him (McGranahan)"
    \key f \major
    
    \time 4/2
    a4( c2) bf4 a2 g | f1. \bar "||" \break

    \time 12/4
    \partial 2.
    f4( g) a | bf2. d2.~ d d4( c) bf | 
  }
}

% Meter remains quadruple.
% Beat note duration changes with change from simple to compound
% meter.
% Anacrusis completes the measure in terms of note duration rather
% than beats.
\score {
  \new Staff \relative b' {
    s1*0^"Sons of God, beloved in Jesus (McGranahan)"

    \time 4/4
    c4. b8 a( b c) d | c4 b c4. \bar "||" \break

    \time 12/8
    \partial 8
    g8 | g4. e c'4.~ c4 g8 |
  }
}