Concat fix

[vhavlena]

This PR fixes the concatenation problem.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (069554c) 72.94% compared to head (e555504) 72.95%.
Report is 1 commits behind head on devel.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel     #348   +/-   ##
=======================================
  Coverage   72.94%   72.95%           
=======================================
  Files          33       33           
  Lines        4144     4145    +1     
  Branches      955      955           
=======================================
+ Hits         3023     3024    +1     
  Misses        740      740           
  Partials      381      381           

Files	Coverage Δ
src/nfa/concatenation.cc	92.18% <100.00%> (+0.12%)	⬆️

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[Adda0]

(Preallocates delta to have all initial and final states in delta as well. Only then you can append to delta.)

LGTM.

[Adda0]

The rest can do their review later and we will incorporate their suggestions in later PRs.