Fix `modify!`; return a `Pair`

[codecov[bot]]

Codecov Report

Merging #6 (d05360d) into master (11d5f57) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #6   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           24        24           
=========================================
  Hits            24        24           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core.jl	100.00% <100.00%> (ø)

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