## if `do` is not enclosed in `( )`, variable declaration is prohibited
do { | k | set k 10 };
do | k | {set k 10;}; ##ERROR: syntax error - variable declaration disallowed

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## if `do` is not enclosed in `( )`, it supports only the limited number of expressions.

do ; ## this is ok

do 1; ## this is ok

do { set k 10; printf "k=%d\n" k; } { set k 20; printf "k=%d\n" k; }; ##ERROR: syntax error - more than one expression after do