Math macros using a table cannot be called multiple times

[Omikhleia]

MWE (in Djot)

$$`\def{icol}{(\mtable{#1}{#2})}
\icol{a \\ b} = \icol{c}{d}`

(I wanted to check the syntax equivalence)

Expected

Observed

Crash in ./packages/math/base-elements.lua = bad argument #1 to 'gmatch' (string expected, got table)

Analysis

The columnalign option is modified (parsed to a list, and extended to match the actual number of columns) without having (shallow-)copied the options, and the passed options object the same (from the macro), so one invocation affects the other.