Left Shoe Underbar ⊆
Monadic Left Shoe Underbar means
⊆ 'this'
┌────┐
│this│
└────┘
⊆ 'this' 'that'
┌────┬────┐
│this│that│
└────┴────┘
Dyadic Left Shoe Underbar means
1 0 0 1 1 ⊆ 1 2 3 4 5
┌─┬───┐
│1│4 5│
└─┴───┘
1 1 2 2 2⊆⍳5
┌───┬─────┐
│1 2│3 4 5│
└───┴─────┘
' ' (≠⊆⊢) ' many a time'
┌────┬─┬────┐
│many│a│time│
└────┴─┴────┘