Modified Assignment
From NARS2000


A is a name with an existing value, I is an arbitrary index to A, f is an arbitrary dyadic function, and R is an arbitrary array. 
For example, A+←1 is shorthand for A←A+1, and A,←' ' is shorthand for A←A,' '.
a←⍳¨⍳4 a 1 1 2 1 2 3 1 2 3 4 a[3 4]⍴←a[3 4] a 1 1 2 1 2 3 1 2 3 4 1 2 3 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4