The focus of attention in working memory

Working memory is the part of the cognitive system that holds the information we are currently thinking about. Thinking often involves the mental manipulation of one mental object while holding others in mind unchanged (eg, mentally simulating moves in chess). Evidence points toward the existence of a focus of attention in working memory that selects one element at a time for processing.
The project addresses the following questions about the nature of this focus:

Is it structurally limited to one element, or can it take in several elements? How does the focus process relations (eg, verify that in the word "MEMORY" the "E" comes before the "O")?
How are working memory contents updated by the results of manipulating a mental object (eg, in counting, how is the previous value replaced by the new one)?
How does the focus of attention select an element in working memory?
Can the focus be regarded as the entry gate to a bottleneck for cognitive operations that executes one operation at a time?
Can the focus be expanded through practice?
The long-term goal is to understand how working memory subserves reasoning by manipulating elements in structural representations.