An accumulation function {{MATH_18}} measures the signed area in the region {{MATH_19}} between {{MATH_20}} and the {{MATH_21}}-axis. Hence {{MATH_22}} is playing the role of a ``place-holder'' that allows us to evaluate {{MATH_23}}. On the other hand, {{MATH_24}} is the {{CMD_25}}specific number{{CMD_26}} that we are using to bound the region that will determine the area between {{MATH_27}} and the {{MATH_28}}-axis, and hence the value of {{MATH_29}}.