12345 +67890 ------- .
The first production, Start-Problem, applies when the goal is to do an addition problem and no column is being focused on. It focuses attention on the ones column. It also sets a subgoal to process the numbers in that column. The next productions, Read-Number1 and Read-Number2, read the numbers from the column (later in Chapter 4 we will discuss the issue of how ACT-R can actually read information from a visual array). The fourth production, Add-Numbers adds these numbers by retrieving the addition fact that 3+5=8 and sets 8 as the answer to be written out. The fifth production, Mark-Carry, retrieves 0+8=8 and marks 0 as the carry and 8 as the number to be written out. Then the production Write-Answer writes this out and pops the goal of processing this column to return to the original goal of solving the whole addition problem. Then Next-Column switches attention to the tens columns and on it goes.